First off, I am not looking for Superhero MOD, so don't suggest it.
The CTs is the mutants, and the T's is the filthy humans.
When a client connects and joins the CTs, (s)he can choose between the basic mutants.
If the client joins the Ts, (s)he spawns with an primary weapon of choice, nades, armor, a special pistol (will explain), and a special knife (will explain this too).
The buy menu is disabled.
The mutants don't have any guns, if they don't pick one up, or the mutant has one in the series. They spawn with a melee weapon, though (Knife, hand, baseball bat, depending on the mutant).
It's then the humans job to kill the mutants. Unexpected, eh?
They can kill the mutants with the standard weapons...or...
The special weapons injects the mutant antidote, featured in X-Men 3. (The movie)
The pistol can fire darts, three of them, and the knife is an hypodermic needle (or whatever its called), with unlimited contents.
The basic mutants can take two darts, then they are transformed into humans, and transferred to the CTs. The advanced mutants, though, can take more darts, up to 7, making it a hard time to transform them to humans. Both mutant types can only stand one direct injection, though.
When all the humans, or mutants are dead/transformed, the round ends, the transformed mutants get back to the CTs. When 5 rounds have passed, the players switches teams.
That's basically my ideas, if you have any questions/notes/suggestions/comments, please post.