I like the sound of it so far! If I may suggest/ask some things....

Firstly, and I don't know if you were already doing this (it was kind of hard to tell just from the one paragraph), but I think it would make sense to say that only a certain number of children retain their ability to produce imaginary friends during their transition from a baby to an infant. (Just thinking about the number of scribbles that were in that closet at Foster's makes me think all babies probably have the ability, but you could just as well say that only a certain number of children have the ability at all. Babies probably have volatile imaginations just as well-- like that girl Goo who kept making like a ton of imaginary friends for no reason, so maybe gifted babies are just prone to keep cranking out scribbles. Either way, I think it would make sense. Perhaps, in addition to that-- there is an extremely rare number of people who retain the ability into adulthood as well? 

Also, is there going to be a cap put on how many imaginary friends regular people are able to make? It never goes that in depth in the show, obviously, but for an RP I think some sort of limit would have to exist....

This is more of a question, but also-- would I be correct in assuming that the governement would use imaginary friends as more than just weapons? I mean-- with all that expansion, they'd need a way to keep up so they would probably be enslaving imaginary friends as workers, right? 

I think this is a really cool sounding idea so far and I'm excited to hear more about what you're planning.