The way I see it, there are multiple possibilties for baddie teams: Dementors, Werewolves, Slytherins (because they're
always bad
), Death Eaters, Inferi (although theses are sort of tied with the Death Eaters), Giants, Goblins, Trolls and possibly even the Inquisitorial Squad from book 5.
Obviously, they're not all teams, but I can honestly see Dementors as being one of the teams. That way, Harry could be like a protector against them with his Patronus.
I wouldn't be surprised if the Werewolves weren't actually a team to be honest. Quite possibly, Fenir is a one man team, and he might be able to 'recruit' more players by biting on a certain night or something. Or he might not. I dunno.
The Slytherins, I doubt as being a team. I can imagine more one man baddies that are Slytherins, though I can't imagine them as a team.
The Death Eaters are the team that I believe might exist, after the Dementors. If the Aurors are actually roles, then they could be the ones to protect against the Death Eaters.
The Inferi, I can't imagine as a team. Maybe a power that the Death Eaters might use, say... rezzing someone and bringing them back as an Inferi, where they force them to do what they wish.
Giants, again I can't imagine them as a team. Possibly a one man team, though not a group of people. Or, Ammer may have decided to include good ol' Grawp into the game, in which case, the Giant may be good, possible even a vigilante type role.
Goblins and Trolls aren't exactly featured Harry Potter characters. They're mentioned often, but they aren't exactly a focus point in the stories. They're probably not in the game, though I wouldn't rule out the possibility.
And the Inquisitorial Squad... I doubt such a team exists, though again, I wouldn't rule it out. I just can't imagine school students, even the Slytherins, being an evil team.
That's my 2c on possible baddies. Please understand though, that these are just assumptions. I'm just trying to see how Ammer may have worked Harry Potter characters into Werewolf.