I'm not talking just random throw your favourite characters together here, I mean a team that's appeared in comics, even if it's just a one-off team up, or a past avengers or x-men team that you'd like to see function as a unit onscreen...
Personally I'd go for...
Thunderbolts - Red Hulk, Punisher, Elektra, Deadpool, Venom(Flash Thompson)
or
New Fantastic Four - Wolverine, Spider-Man, Grey Hulk(aka Joe Fixit), Ghost Rider
and also the original five X-Men in an X-Factor Investigations scenario
Personally I'd go for...
Thunderbolts - Red Hulk, Punisher, Elektra, Deadpool, Venom(Flash Thompson)
or
New Fantastic Four - Wolverine, Spider-Man, Grey Hulk(aka Joe Fixit), Ghost Rider
and also the original five X-Men in an X-Factor Investigations scenario