I don't think forcing further requirements onto the rest of the player base would be a popular decision. Remember at the end of the day this is still a game, and having a bunch of variations of a single unit type can be very confusing to players.
Well yes but we already have that confusion with engines so there really is no limit as to what can happen. Like I said before, if we can have 7 different types of engines then we can have 4 ladders