- An abduction mechanic. The most easy way to do this is to give the fairies a slave UU. Hunters and Trackers (or their respective UUs) could have different ways of getting those. Summer court hunters (Changelings) could, for example, sacrifice themselves in an opponent's city to create two of these slaves in your capital. Winter court hunters have a certain chance to generate a slave in battle. Et cetera.
- Those slaves can upgrade to the melee and archery lines. Units upgraded to those should be weaker, especially when attacking cities, but should cost reduced support or something. The Fey should have no way to actually build those lines, just upgrade to them by way of abductees. The Pixie/Sprite line would be the replacement for your mounted line then, and should behave at least partially like it (withdrawal chances, increased movement, the works - iirc, they already do a bit). Make a chariot UU that behaves a little more like melee and all is well.