I think the Maori and to a slightly lesser extent the Mali are well designed Civs in terms of balancing a unique gameplay gimmick with a viable strategy.
I think the Kongo's abilities are a bit too parasitic with other mechanics for their gimmicks to actually be considered "well designed", even if the effects they get are themselves powerful. I personally wouldn't want to see more civs that are both narrow in their application and locked out of major aspects of the game. Consider Civ5 Venice. The One City aspect and trade bonuses they do get are certainly interesting, but I don't consider them well designed due to how many constraints are placed upon them.
In the case of the Maori, they have an unconventional start which is offset with strong bonuses as compensation AND they have effects afterward that still manage to subvert several conventional city building. So you still get the advantages of having an over the top gimmick without being completely locked into that initial gameplay hook. For this reason, I would prefer unconventional gameplay use the Maori as something of a template rather than the Kongo
I just think it is highly unlikely that they would release large scale mechanics like that in a piecemeal DLC. It just doesn't seem economical to have cool new mechanics to work with, but only one maybe to civilizations that get to take advantage of that. They could decide to reworking existing civ options in order to make use of those mechanics, but if that is the case I don't see the point of making it part of the DLC versus a general update.
That's why I'm leaning so heavily on precedent because besides the possibility of this being a third round of content release, I haven't seen any indication that this will be much different from previous releases. I am basically trying to set up any expectations I have based on the lowest common denominator.