If there is ever going to be a campaign mode, I don't think it'll happen in this game as it isn't setup for it. More than likely if this game sold well enough to warrant a sequel then we might have a shot at it if the demand is high.
It's funny that you compare it to WoW because I was having the same thoughts. Playing it made me think of how fun it would be to have an arena-style multiplayer as well. 2v2's, 3v3's, 5v5's.
Campaign with a chomper tanking, sunflower healing, cactus/peashooters doing DPS. Or all-star tanking, scientist healing, etc...
Either way, I feel like the game is already well-balanced for it. Especially the arena-style games. Very small maps where the teams have to use great coordination to win. I'd love it.
