A common suggestion for the plot of StarCraft III is that the UED comes back for revenge and everyone unites to destroy them, just like they did for the last three big bad evil guys, ushering a new era of peace for the galaxy in which terrans and protoss and zerg can all sit around a campfire and sing kumbaya just like they did at the end of StarCraft II. I am taking that at face value and assuming that StarCraft III will be titled "The UED Strikes Back", "Revenge of the UED" or whatever.
What about StarCraft IV? Who will be the big bad then? The Overmind resurrected? A new Queen of Blades? Duran or Amon resurrected? The UED's third fleet? Another xel'naga who survived? A member of a fourth race who wants to build the hybrid to destroy the universe?
What about StarCraft V through StarCraft IX? What will be the plots of those? Who will be the big bad evil guys? What will be the new motivations of the terrans, protoss and zerg? What new retcons will invalidate all the previous entries in the franchise?
The history of WarCraft shows that every new entry essentially in a Blizzard franchise retcons everything that came before and expands the scales by making up more of the world. WarCraft I was limited to the orc invasion of Stormwind. WarCraft II introduced the northern kingdoms of humans, elves and dwarves. The WarCraft II expansion introduced the orc homeworld. WarCraft III introduced the continent of Kalimdor and the Undead Scourge and the Burning Legion. The WarCraft III expansion introduced the naga and blood elves.
Similarly, every new entry in the StarCraft franchise has expanded (and shrunk) the world further. The zerg and protoss were previously established to have galactic empires only for later games to reveal their home planets are literally a shuttle ride away from Korhal. The xel'naga were expanded from vain scientists to an evil shadowy cabal to space gods. The zerg changed from a devouring swarm to peaceful shamanistic hippies (just like the orcs). The protoss lost their ancient culture in favor of becoming ninjas. New factions were introduced solely to replace the aspects that were destroyed.
As always, there are numerous retcons and inconsistencies added in every new installment. These accumulate until the setting becomes unrecognizable past the surface.
Rather than try to speculate game by game, I will just speculate about specific plot points and retcons that will inevitably show in the sequels.
Plot point speculation: the revenge of the UED
The UED arrives to take revenge for the loss of their previous fleet. Either they will be white knights who die for being too good for this sinful franchise, or they will be generic unmemorable genocidal lunatics. Either way the Earth will be eaten by zerg because that was planned for Brood War before the budget ran out.
Plot point speculation: the heroism of Amon
Amon will retconned as being the good guy all along, just like Sargeras. He was actually trying to stop a nebulous antagonist who wanted to kill everyone. Either he was never trying to kill everyone or was somewhat corrupted from a formerly white knight.
Amon's enemy is either a pre-existing character (Overmind? Khas? Ouros? Duran? Who knows!) or a new one made up for this game and then retconned into the back story.
Plot point speculation: dead characters will come back to life with technobabble
Stukov was a decomposed corpse when the zerg found him, but he was restored with all his memories intact even though that should be impossible. Nobody in this franchise will stay dead if the writers want to bring them back. World of WarCraft brought back obscure characters like Teron Gorefiend and Little Timmy, only because the writers found them while reading the plot dumps to find stuff to reference, so the Overmind and Zasz and Daggoth are definitely coming back in the future.
For that matter, any dead character could be resurrected through technobabble. The zerg could resurrect long dead corpses through their technobabble. The Overmind was resurrected despite being killed with a plot device, and its genetic material is still preserved in all zerg larvae according to Raynor's techie friend. The protoss canonically have time manipulating capabilities, so the writers can easily twist that into casual time travel. Souls and the afterlife canonically exist, so expect that to be invoked.
Plot point speculation: the enemy of the xel'naga
The xel'naga will be revealed as having an enemy race of space gods. The narrative will flip-flop over which race is evil or not depending on the needs of the current game. Everything in the franchise will be retroactively the result of these two space god races duking it out for control of the universe.
Plot point speculation: new factions that should never exist will be added
Brood War and StarCraft II both added new factions that had no support in prior lore. This generally contradicted the themes established previously (the UED added when the SC1 was supposed to be a clean slate), exist only to replicate an aspect of the three races lost through retcons (the primal zerg represent the SC1 zerg motive, the taldarim represent the evil protoss), or were added because the writers could not be bothered to use an existing faction (Umoja and KMC, plus the protoss tribes and zerg broods, are still non-entities) or just because they could (Moebius, Purifiers) or as skins to sell in the war chest.
I fully expect that the SC1 Protoss Empire, Terran Confederacy, and Zerg Swarm will be reintroduced in their entirety as new factions using very flimsy reasoning and in outright defiance of other retcons. The writers could justify it any way they wanted to even if it did not make sense.
A galactic protoss empire with tribes and castes and the Khala that we never heard of, despite the BW and SC2 retcons, introduced as a new faction? The flimsy justification is that Aiur is actually a lost colony that forgot the truth or something.
Another Terran Confederacy that oppresses its people? Koprulu was a lost colony of that Confederacy. Maybe Earth is a lost colony, too!
Another Overmind who wants to consume humanity's purity of form to fight this other protoss empire? They are the true zerg and the primals and Amon's corrupted zerg are just their degenerate descendants. They have the same broods as SC1? The zerg leaders casually clone themselves endlessly so they can never be rid of (which is great for endless sequels).
Plot point speculation: when taken together, the story will not make any sense
Every sequel has introduced retcons and pointlessly changed character motivations in a way that renders the plot of the series nonsensical. It does not make sense that Amon only took over the Khala in LotV or that he did not feed the Taldarim to the Overmind back on Zerus. His entire backstory requires him to make nonsensical decisions just because if he did something remotely logical then the pre-retcon story never could have happened.
Similarly, the retcons introduced by the following sequels will only exacerbate this problem. Looking back over the previous games will show the characters acting in nonsensical ways because otherwise the plots of the previous games could not happen.
Plot point speculation: every plot will be resolved with a nonsensical deus ex machina
In SC1, the Overmind was killed by a deus ex machina. It was foreshadowed, but that doesn't change it from a deus ex machina.
In BW, Shakuras was protected by a magic temple with no explanation for why it could hurt the zerg. Kerry won because of writer fiat even though her allies knew her plan the whole time and should have prepared.
In SC2, Kerry was de-infested by deus ex machina (despite nanomachine cures already existing), and killed Amon using another deus ex machina to make her xel'naga (even though he was previously established as powerful enough to kill all the other xel'naga himself).
I similarly expect every single game will be resolved by a lazy deus ex machina rather than the players just fighting really hard.
Plot point speculation: minor characters from the written fiction will appear, but their characterization will be butchered
A number of characters from the dark templar books appeared in SC2, but they were just those characters in name since the game writers clearly never read the actual books.
I fully expect this will happen again and again when the writers cannot be bothered to create original characters. (Conflating Artanis with the SC2 Executor is a classic example.) This is exactly what happened in WoW.
In fact, I expect that child characters will have implausible ages. (In WoW, Sylvanas' niece and nephew appear as adults despite them being born sometime in the WC3 era. Additionally, the kid from Blood & Honor showed up as an adult, even though he was born shortly before Thrall took over the Horde.)
Plot point speculation: infested protoss will appear
Yes, Metzen said this was impossible, but he isn't working for Blizzard anymore. They will ignore him for the sake of gameplay. This will either be justified with a flimsy excuse about how a new method was developed if it is ever justified at all.
Plot point speculation: past events will be glossed over and misinterpreted by writers
The plot of Brood War was ridden with holes. When it was recapped in SC2, pretty much everything was glossed over. The recap in WoL simply ignored the plot holes and hoped the reader would just assume things happened in a way to make sense. The recap in LotV actually seemed to re-interpret the events of SC1 differently by omitting key details and mentioning events out of order. The LotV recap seems to make the plot of SC1 entirely about Raynor and Tassadar fighting their respective evil hierarchies, when the original was very different (and it completely ignores the forgotten zerg cast, RIP).
In addition to deliberate retcons, there will be numerous accidental retcons due to the writers either being ignorant, pressed for time or not caring about the details of past games and fiction. This is forgivable since we need a wiki to keep track of things, but then why does Blizzard not build a team dedicated to ensuring that continuity is maintained like Disney does for the Marvel movies?
Plot point speculation: distance and time will be meaningless
Remember how in BW, the UED traveled from Earth to Koprulu instantly and the cut plot involved Earth being eaten by zerg? Remember how in SC2, Aiur, Zerus, Ulnar and pretty much everywhere else were literally a shuttle ride away from Korhal? Remember that Aiur was not originally located in Koprulu, but that Koprulu was on the frontier of the Protoss Empire?
Rather than adding other explanations like a warp gate network lying around, Blizzard has consistently treated distances and travel times as meaningless. The characters can travel wherever they want instantly, making the galaxy seem really small.
Plot point speculation: history will be prone to change to the point where keeping track is pointless
Every installment has rewritten the backstory over and over.
Koprulu was previously severed from Earth, then changed to being secretly spied on, then finally an unruly colony of Earth (cf. Heroes of the Storm, Stukov biography).
The protoss were originally limited to Aiur during the Aeon, then custom maps and such said the Aeon covered an arbitrary number of planets. The protoss originally had an empire with multiple species, then it was changed to a fallen empire, then it was changed to their homeworld being in Koprulu on the frontier of their empire (how?!).
The zerg stripped their burning homeworld in the galactic core to a lifeless rock, then it was suddenly in Koprulu with a temperate jungle.
Plot point speculation: Blizzard will never do the smart thing and reboot everything
Even though the retcons Blizzard makes all the time weaken the story and render it nonsensical, they will never simply reboot the franchise. Long time fans will complain about the retcons but when the possibility of reboot is brought up they will refuse to consider it, or only accept a reboot at a certain point in the series (typically after the first game they played and developed a nostalgia filter for while hating the subsequent games and their retcons). They will come up with numerous excuses to justify their hypocrisy like "let's just set the sequel in another sector" even though that solves none of the problems we complain about and only exacerbates the problem with more retcons.
StarCraft 3, 4, 5, etc will come out and consistently prove my cynical predictions. Although the stories will be consistently terrible and the lore will become increasingly unrecognizable as retcons accrue, the competent skirmish and multiplayer game design will ensure the franchise continues to make money even as everyone following the lore complains that it sucks (like they do for WoW right now). Whenever I argue that we should reboot everything, as I will continue to do for as long as I am in this fandom, the people I talk to will complain that should not happen even though by this point Blizzard has shown they will never produce Brood War 2 or Wings of Liberty 2 or what have you like some fans wanted them to.
Conclusion
What do you predict?