The closest games I can think of on the AppStore in the same vein as King's Bounty or the Heroes of Might and Magic model of gameplay are:
Battle for Wesnoth
Palm Heroes 2 Deluxe
Trying to think of the other awesome title on the AppStore that has IAP expansions with a cool style but the name is escaping me...
And of course King's Bounty: Legions which is currently on the AppStore!