In SC1 we were told that the Zerg had assimilated countless species as they traveled the galaxy. Each cerebrate would develop a personality around their directive and alter the makeup of their brood to better follow that directive. In SC1 the only example we saw of this was that Tiamat breeds were stronger than typical (Jormungand?) breeds; in SC2 the Leviathan breeds specialize in aquatic warfare.
I think that all the breeds introduced throughout the series could serve a role in the extended swarm. Some of them are so similar that they would need some kind of tweaking to stand out. (A number of differences in army compositions between SC1 and SC2 only make sense in terms of game mechanics rather than fluff.)
I do not see much difference between corruptors and devourers; funny enough the beta version of the corruptor had the ability to infest enemy air units and turn them into static turrets. Brood lords, at least in comparison to guardians, spawn broodlings.
The difference between creep colonies and the tumors and crawlers seems minimal, but of course in the SC2 beta the Zerg had a completely difference set of defenses. Neither seems clearly superior to the other due to mutual trade-offs: creep colonies spread creep and defend but cannot move, crawler defend or move but cannot spread creep or survive long outside creep, creep tumors spread creep but cannot move or defend.
The scourge nest in HotS displays a dramatic difference in the way a breed is spawned; funny enough, in the SC1 beta each Zerg structure produced its own larvae that would morph into the breed the structure was named for before later iterations centralized larvae production at the hatchery while leaving the names untouched (hence why the spawning pool does not spawn anything).
The queen underwent the most dramatic change between games, to the point where they share nothing but a name; the infestor bears the closest resemblance to the old queen. I suspect this is a casualty of the fluff conflicting the mechanics: in the SC1 game only overlords provide control and queens require a queen's nest to spawn, whereas in the fluff queens control laborers and overlords control warriors. Furthermore, the beta versions of both queens were dramatically different from their final versions (the SC1 beta queen has glaive wurm, dark swarm and plague; the SC2 beta queen built various base defenses).
In terms of breeds which you could expect to exist in the lore without appearing in the game armies, I can imagine quite a few things. For example, I can think of a few laborer strains beyond the generalist drones that specialize in specific tasks: breeds that dig nydus tunnels, breeds that farm creep (a la the SC2 beta queen), breeds that only harvest resources, and breeds that only plant structures, all managed by the telepathy of the queens.
I can think of a few different ways to mutate the creep colonies beyond stuff like crawlers. You could have colonies which replicate certain unit attacks like hydralisk spines, lurker spikes, spawn broodling, dark swarm, etc. You could have colonies which function as mini-hatcheries, producing larvae and/or accepting resources. Maybe, I don't know, some kind of bunker colony which can host multiple crawlers, or a expensive siege beast which attacks like a spine crawler without rooting and an impaler colony when rooted.
What do you think? Is the swarm really more expansive than the games would lead us to believe?