I recently heard that Groot can "assist" a certain character (I'm not going to outright leak anyone by name), but it was really vaguely worded and I'm not sure if MvC1-style assists will be revealed at a later date or if this specific character has Groot summon moves. Either way, it doesn't sound like he's playable, but we'll see. (Don't take this as confirmation of anything, but I thought it was worth mentioning.)
If Groot summons are part of Rocket's special moves and groot isn't an actual character.
1) That leaves an extra slot for a dream pick.
2) Cloak and Dagger should be added as one character.