I mean, I guess I could see adding taxxons and yeerks, but for one thing, the gameplay would be very lacking. You'd have to return somewhere every 3 days as a yeerk, and gametime goes by much faster. Where's the logistics there? Plus, you'd have to wait in a yeerk pool to infest someone anyhow, so you wouldn't really be a yeerk, right? You'd be whatever race you infest.
And as a taxxon, I mean, they are really useless, unless they come with an ability to create several other, swarming taxxons around them at any time. Because just about ANYTHING can kill a taxxon.