I just had a completely crazy idea: to recreate castles along with the old units. For example, for the Elven city: we prohibit Centaurs, Pegasi, Dryads, and Dragons in the city, and place dwellings for Fairies and Phoenixes near the city, protecting them with a Guardian of Questions, who will ask for money and resources (this will be like an analogue of building them, since they cannot be built in the city).
And how would you upgrade them? Place a Fortress nearby on a hill?
The idea is interesting, but it's easier to make a mod. Because:
1. It will be a hassle to go around all the dwellings every time, and then upgrade the creatures.
2. There will be problems with morale.
3. The poor computer, which is already stingy with bright ideas, will drool and start malfunctioning :D