Yeah, RTR has the Area of Recruitment (AOR) system, which is similar to what you are talking about, although the troops are available immediately. SPQR has a different take on it, with the gradual 'Romanisation' of provinces providing different troop types.

Going back to MTW, MedMod did this first (as far as I'm aware) - it certainly added new challenges.

I'd welcome this added level of detail to the 'vanilla' games, so hopefully whatever the next TW game is, it will have this feature.