Trouble with that angle is it makes the UED look like the good guys, when they are most definitely not good.
There's actually no basis for saying they're not good. We hardly know anything about them. They're humans, humans are humans, how are they different from the Terrans from the Koprulu sector? In human nature, they probably aren't. We can't say they're either bad or good because there are many people who are good at heart and many people who are bad.

Anyway, if the UED came to combat the Xel'Naga and if they won, it doesn't automatically make them look good, they're pretty xenophobic. Also, in response to perfecttear's post, I highly doubt they'd establish peach with the Terrans, instead they'd just take over.

It's been said before in this thread and elsewhere, but I hope the UED stays out of the picture. There's plenty of universe to be explored without dragging boring old Earth into things again.
Personally, I found the Terran campaign in Brood War more interesting than in SC1 but that's just me, I love Earth.