First and foremost, welcome to Modding Union
As you've fairly put it, map-specific units are not only an intriguing addition to the game, but also an opportunity to give life to something that, otherwise, would not have the proper chance to be (due to balance-related issues, mainly). Said that, I believe it's not at all an evil to suggest some types of exotic/extravagant units which could fare well in the Mod.
My only concern, though, is the supposed bonus and effectiveness boost that the target faction then acquires by the inclusion of additional features, whether it be units or heroes. That is, I presume that one of the challenges entailed by such proposal would be to ensure that one of the two or more parties involved will not enjoy so generous an advantage, to the point of breaking balance altogether. Plus, I feel like remarking, it ought to be noted that special units are named so for the exact fact of being 'special'; hence, a limited number of them is another strong yet needed limitation, methinks. A proliferation of them would just defy the founding purpose, I would say. However, keeping things unique would be more than fine to achieve the said goal.