The undead unit, after dying, would leave a corpse like the tame lion (which wouldn't be too hard for the game engine). It would then be convertable by any Necromancer, regardless of what player. It is converted through revival, and each revive costs an amount of something like the priests' faith, which can be replenished over time.
The necromancer should, IMO, be allowed about 2-3 revives before having to wait and replenish. The undead unit can either be summoned the the necro, or summoned from a 'building' called a cemetery. The necro idea could possibly be extended to every single unit being revivable and having no undead unit. (But this could take a whole lot more work for ES) The necro's second skill could be healing friendly units, and attacks enemy units with a low melee damage attack. ---- A small solution to flying units: A unit like the griffin, for example. Perhaps have it fly while moving around, but when attacking, it lands on the ground and starts melee clawing its enemies. ---- There was some other thing I was going to suggest, but I suddenly forgot. - Miff