Thats not a bad idea actually. I generally do similarly in campaigns that I play, but I usually just try to use defeated enemies depending on who I'm playing as. Generally its KH, Pontos, and one of the Gallic factions.

In my last game as Epirus I got to the point of being a super power with a navy and army large enough to interdict peace between my enemies and the KH who i had decided to use to represent neutral and allied independents. Since I knocked KH out early they were perfect for "secondary eleutheroi", especially after I destroyed most of the Ptolemaic armies, but didn't want to eliminate the faction completely. I more or less pinned them in, then got to sort of rp reasons for protecting the KH, Ptolemaic rebellions, Pax Epirus etc. Even though I wasn't allied with the KH, they still worked as buffer states quite well, since the 6 or so cities they had weren't really connected they never seemed capable of mounting an offensive.