I've got a few ideas. One thing i hated about this game is the one-tracked repetitive levels.
Like many have mentioned before there is a whole world to explore, so why not incoorporate that into the game. The levels could be made bigger and more vast, with alternate routes to explore, instead of the same old rooms and the same old path you must take. Perhaps even splits in the path, where the team can choose to split up or stick together. If they split up, then the challenges would be greater, but so would the rewards.

Also, there could be more intelligent badguys, that attack in more than one or two ways. Even the standard Booma has a huge mouth on it. There is no way that it wouldn't attempt to bite you or tackle you to the ground.
Also, Chaos Bringers are pathetic. There is one hell of cheap way to kill them quickly and easily. Perhaps they could unleash a flurry of blows, or kick you with their front legs. Stuff like that!