Finally decided to take my suggestion and run with it, eh? I approve. Plants and animals though? Animals might work as a sort of skinwalker-type class, but I dunno about plants... from a fluff point of view that is. I dont know if I like the eldrich horror thing. If that's your game, go have a look at the ozodrin, its better equipped for that, but this... This has the potential to really rock as something else if you're careful with it. If I recall correctly, the idea was to take the binder and make the stuff you bind less powerful, but allow more things to be bound at once, so this basically becomes an entire magic system to itself.
I think the key here isnt variety though, so much as focus on a particular type. There needs to be a way to regulate what you could bind at once, so you didnt have people binding demons, celestials, fey, and the ghost of grandma Gertrude all in the same body. That gets kinda silly and probably opens the class up for a lot of exploitation and munchkinism (Not to mention the likelyhood that the character will die when the different factions start fighting each other inside him) and I think overall this would create more limits on what the binds can grant than variety from what could be bound. I know playgrounders dont tend to like limits, but in this case I think its a good thing.
So for instance if you wanted to play a guy who regularly allowed himself to be possessed by demons for power, you could focus on demons and do that. Or a guy who was haunted by the (literal) ghosts of his past, you could play him as focused on undead and being regularly possessed and controlled (as an rp element done by the player) against his will. Perhaps a holy man that acts as an earthly vessel for his divine patrons, allowing celestials to work their will through his body. Maybe a friend (or target) of the fair folk who, due to some blessing (or curse) can call upon the grace of the fey (or have the comehither put on them) in times of need (or when most inappropriate). This has massive roleplaying possibilites.
Basing this on the alignment of the bound creatures might be the easiest way to manage it, but type might be more reliable. If I thought that it would get used (unlike most of the stuff I make for our group), I'd tackle this project head on myself out of sheer interest. For now though, im content to sit back and watch what the playground makes of this. I hope it generates a lot of responses, since the demon binder you made turned out so well. I'd love to see it expanded into an entire magic system.
Note: Edited for clarity of point