It is a bit difficult at this point (43 official civs just for AoE II) not to try to think of the long term. Indeed: do we want eventually 50 civs? 75? 100?
On the one hand, it is always more diversity and experience for the historical players, who are most of the people who would purchase a new DLC.
But on the other, it makes for a game harder to read for new comers. It is also harder and harder for devs to come up with new ideas on how to make a civ unique without making the legacy civs we all know and love feel blend. Of course, they can just keep changing the old civs, but this would go against the initial spirit of DE, which was supposed to be: ‘let’s balance everything existing once and for all, and then add new content adapted to that state of balance’. It took a bit of time, but I think devs did a marvellous job with that, mostly.
Moreover, more civs also means more mechanics probably, some of which generated a lot of passions (one-time techs, charged attack, etc.). And I think quite understandably so, as we want to have AoE II keep its identity, which very much means its core mechanics, without adding anything too ‘alien’. Not that I don’t like change, but then I would rather play another game.
Another issue with always adding more civs is the cultural representations which belong to AoE II. Of course hot topic, and in a way a lot of things are already inaccurate from start (Aztecs with trebuchets, etc.). I will just keep my opinion to this: I don’t think we should dig for civilisations so much that we start cutting existing civilisations which are a legitimate representation of a continuous culture into tiny pieces, like some here want to do with Germans (Teutons), Slavs (which was too broad to start with, but is now fine representing eastern Slavs), Italians (I guess Venice would not be the worse, but we already have Sicilians, Byzantines, and now Romans), or Chinese (which is broad indeed, but China was unified during a large part of Middle Ages, and this does not mean Tibetans, Jurchen, etc. can not be represented). Adding Burgundians was already sort of borderline. Moreover, there is a number of civilisations that did not form kingdoms/empires or even states, or that were centred around completely different activities than warfare as it occurred in Europe, the Middle East and Asia. And so it is fine not to add them to the game. For instance, I think Polynesians did marvels in navigation, and formed a number of original and interesting civilisations at an era we call Middle Ages. But does it make sense to them to AoE II? I don’t quite think so. It may seem eurocentric to some, but I think it’s fine having a game mostly centred on Eurasia, including the Middle East, the Mediterranean and India, which does not exclude kingdoms and empires from all over, as much as they were somewhat centralised states and went at war. So I am only happy the Malians and the Aztecs for instance are part of the game, and cultures like the Chimu or the Kongo seem like they could be appropriate additions.
All of this to say: knowing which new civs to add is a tough, tough call. This actually encouraged me to write another topic, to suggest non-civilisation possible DLCs.