Isn't more likely that rocket w/ groot would be on the cover of the groot variant while star lord or another gotg will be on the variant cover for the gotg issue?
This of course assumes these 20 variants even confirm lead characters from those series being playable.
I think you're missing that Rocket's got a book in there with its own variant cover.
Out of all of the covers there, Groot's could be the biggest wildcard, since it isn't Groot. Its probably tangentially related to him. Since the game is 2v2, I think it could be a cosmic villain like Ronan to sort of give Thanos a partner. Then again, it could be Gamora (who's playstyle probably would be quite different from any of the other speculated Marvel characters) who could team up with a pottential Rocket, Star-Lord, or Thanos. Maybe Captain Marvel too really. Or it could be Shuma-Gorath. Who knows.
While I don't see the solicits for August in full, one thing that is somewhat odd is the Defenders variant. Daredevil, Iron Fist, Luke Cage, Jessica Jones, and Punisher, who are the members of the Defenders for this book all have their own ongoings, so maybe whomever gets represented on that cover actually isn't them. Then again Captain America isn't for his own book, and is probably either for Secret Empire or Avengers.
To that end, outside of the X-Men/FF, looking at the July solicits as a reference, there are a bunch of books that would get characters if those characters were in, but it appears that isn't the case. Squirrel Girl, Gwenpool, Nick Fury, Wasp (Ant-Man would probably go here), the Inhumans, the Secret Warriors, Ms. America. Ms. Marvel's book is also skipped, but if anyone's representing Champions its probably her. Miles would be under his own book, which does have a variant.
Also, since these are August solicits, regardless if 3 of these are for DLC, we'll know at least 20 (maybe 40) of the roster before September....then again they could just have variants reuse characters.