more evil behaviors and interactions would be great, like recruting minions and team up with other evildoers, maybe working out ridiculous plans to take over the world depending on the sims skills and interest (a genius would maybe want to build a robot army, or spread a virus through the internet, while a creative sim would compose a song that makes everyone do their bidding or something like that) all for the evil endgame of becoming the ruler and than being able to set rules sims have to follow (from ridiculous ones like "no hats allowed", elders aren't allowed into bars to establishing taxes which the ruler would get of course...).
But as the evil ruler of the world, the sims would have to deal with heroic sims trying to take them down, their followers betraying them, the people rising up and such things...it is not easy being evil.
Oh, and a new type of venue "evil lair".
And of course more evilish CAS and BB items.
This all just from the top of my head...I am sure I can come up with more, just gimme time ;)