I mentioned in another thread about how it would be interesting if they incorporated spotpass with the swarm mechanic. So the developers themselves decide exactly which pokemon start swarming in the game.
That would be pretty neat I think, it would shake things up a bit if they used spotpass to download small changes like that. Plus it would mean they could make any pokemon swarm any area they wanted to rather than being stuck with whatever is shipped with the game.
Failing that I would like to see a larger amount of pokemon included in the swarms considering there will be over 700+ pokemon now, they shouldn't be stingey with the pokemon.
They could always make it so the swarming pokemon you get is dependant on the version you play....and maybe if you connect with somebody who has the other version there's a chance that whatever is swarming in their game comes to your game and vice versa. (sort of like how villagers move in animal crossing)