This release made it easier by adding a big data point.
With only three-ish factions, picking one to focus on first would make getting things table-ready more realistic. (Since there's crossover between the good and bad sides with the neutrals.) And really, as the game is so new, I think we can safely speculate as to what's coming out... eventually. Having Knight Models spoil us so early with surprises like the Dark Avengers and the Thing gives us some strong material with which to guess.
Hells to the yeah.
The COSMIC side of things currently has the least number of figs, mostly the Guardians. And really, no bad guys. Adding the Thing and giving Doctor Doom the Cosmic icon leads me to believe they will also have the Fantastic Four and their longtime foes. Armed with that knowledge, I would speculate Cosmic would also get Ronan the Accuser and the Kree, the Silver Surfer, and the Skrulls. And frankly, the possibility of getting to play the Super-Skrull is tempting in and of itself. But who else? Other space types, sure, but even given recent comic book events, that's the smaller group of the three factions we have to pick from. On a farther limb, I would guess the royal court of the Inhumans would fit here as well (with bonus Lockjaw!), and hey, even more obscure Fantastic baddies like the Red Ghost and his Super-Apes. So, some choices, but I don't see a lot of increase right around this corner.
When Juggernaut comes out, this crew will be very tempting.
The next faction being the MUTANTS gives us an even more self-contained world, almost entirely the X-men and their various splinter groups facing off against their foes. There's easily a number of less-popular heroes I could work into a team (Havok, Polaris) considering the depth of their rosters over the decades. And so many of their foes have become their friends at one point or another, so I bet there will be many neutral picks as well. But the Brotherhood has an equally lengthy tenure, and adding in the Marauders, many fun options. Plus Juggernaut! And, dare I hope, Arcade! (OK, maybe that's just me.) I seem to have an affinity for playing the bad guys, to boot.
Finally, the SUPERHUMANS have the widest scope for growth, IMHO. Right off the bat, Black Widow implies to me that anyone SHIELD-related would fit in, which also gives us their enemies over in Hydra. Spider-man is neither a Mutant nor Cosmic, so we get his friends and his foes, including the long-awaited Sinister Six (ooh, yesss.) This also implies, to me, that Superhumans will also get the Marvel Knights affiliates like the Punisher, Moon Knight, and Luke Cage and Iron Fist. The surprise introduction of the Dark Avengers also makes it an easy leap to include the Thunderbolts here. And on top of all that, there's also decades-worth of Avenger friends and foes to bring in, from Quicksilver and Scarlet Witch to Taskmaster (another personal fave) and the Wrecking Crew. My dream pick would be the Squadron Supreme here, and with the Dark Avengers going here, this'd be where they go too.
Personally, I think the Cosmic side will have less attractive choices for me. I could see Dr. Strange getting in there too, but it won't expand that faction much more. The Mutants are perfectly self-contained, and have the most crossover potential (so, so many of their foes end up joining them at one point or another.) They'll always just come back to the X-Men and their enemies, in the end. Which isn't bad! I would love to run some Brotherhood with some classic Marauders backup. But leading the pack, Superhuman has the largest capacity for growth. It's a wide net to cast over anyone that isn't Cosmic or Mutant, and it covers a pretty wide swath of the Marvel-verse.
