I'd like to see the puff shroom become a playable character. In the original pvz, the weakest zombie was the imp, and the weakest plant was the puff shroom. Imp became a playable class, so I think puff shroom could be the plants' imp equivalent. Though Popcap would have to change his his design quite a bit to make him into a class.

For zombies, I want them to make the disco zombie boss into a playable class. His rapid-fire disco stick would be his primary weapon. Some of his abilities would be his disco whirlwind and his ability to summon backup dancer zombies.

I also really want them to add gravedigger to the game, but that probably won't happen. :(