Other than the big pagan/theocracy/republic DLCs, which if I remember the Devs said they'll get to around eventually, I thought the Aztec DLC, while ahistorical, did show that the devs were interested and more than capable of making smaller, minor DLCs that don't count as major DLCs. I hope that later minor DLCs also are a bit more historically focused, and that, ideally, these minor DLC focus on different regions and improve gameplay there. In other threads, I have already made some of these suggestions:
1. Iberia - focus on improving the balance of power between the Christians and Muslims; a Reconquista mechanic for Christian powers, but also a way for Christians and Muslims to show their tolerance (as was possible historically), possibly through some sort of slider or at least through event; and other events focusing on this region
2. Zoroastrianism - I think several fans also like this idea, and it would be perfect for a minor DLC, possibly mechanics for how a player (or lucky AI) might resurrect the Zoroastrian faith
3. Celtic World - focus on flavor and gameplay changes to the Celtic nations - for instance, a more difficult (and realistic?) way to divide fractured Ireland, representation of the monasteries, etc.
4. West Africa - implement the Saharan trade routes, the gold mines, the slave raids, etc., and other flavor stuff and events for West Africa - maybe even more provinces - to make that part of the world a bit more interesting
5. East Africa - balance things out so the Ethiopians and Nubians don't get wiped out, introce the baqt somehow, other game mechanics, flavor, and events
6. Nomad/Turks - nomads and Turks have to choose between keeping their traditional nomadic ways or adopting the cultures or ideas of the conquered (so the Persians in the case of the Turks and Mongols), this might make an interesting conflict
7. Ottoman Empire - I like them, and while it's a late game start, perhaps some additions to siege equipment, the decline of the Byzantine Empire, and the rise of the Anatolian Turks might be interesting
8. Renaissance - extending the playdate by a couple of decades, and introducing mechanics and flavor to represent the Italian Renaissance and for the city-states' struggle
9. Hanseatic Leage - self-explanatory for those who know what it is
And so on. I think minor DLC with a regional-focus would be really fun, because if my impressions of the Aztec DLC are correct, the devs won't need to actually make anything that substantial like Sword of Islam - maybe introduce just one minor mechanic or two for the region in questoin, throw in some flavor elements like titles and events, maybe balance things out a bit, and voila, you got a nice little DLC that doesn't take up too much but adds so much.
In short, I think minor DLC might be a great way to improve various regions of the game without having to resort to huge catch-all DLCs.