In a perfect world half of the civs could stand to have new architecture sets, however the fact that we haven’t gotten any new ones since DE itself I think is a sign that we aren’t likely to get many more (if any) so we need to really consider which ones are really critical versus just a nice to have.
In my opinion a “Barbarian” architecture set for Goths, Huns, and ideally a couple new barbarian civs is a very high priority. Might even be able to wrap Cumans into this, but I’d need to do more research on that.
I’d love the Mongols to get their own architecture, and if they did then maybe Huns should get that, but i’m not sure we’ll ever get an architecture set for just one civ. Maybe adding Huns, Cumans (again i’d need to do more research on that) and a new Gokturks civ (I’d need to research this option too) could make it more feasible.
Persians I know there is a poll to give them the Central Asian (Tatar/Cuman) architecture set. I don’t think this is a perfect solution but its the most realistic one, and its gotten quite a bit of backing.
Spanish/Byz I honestly think the Med. set works decently enough for a similar reason to my Mongols reason; I don’t think they’re going to do a set for one civ. Granted you could give Portuguese this theoretical Spanish set, and Georgians the Byz set, but I still think this is low priority at the end of the day.
Japanese/E. Asian: I think the most realistic solution is to leave the current set as-is to the Japanese, and make a new set for mainland E. Asian; I think the current set is closer to Japanese, and they won’t make a new set for one civ so just leave the current one to Japanese, then give a new, proper one to the other E. Asian civs. This one is a bit higher priority in my mind but still not the highest.
TL:DR Honestly if they made a Barbarian set and a Nomad set (or even just one or the other) and a new E. Asian pack I’d be ecstatic, if I had to choose just one i’d say the Barbarian or Nomad set. Any others would be nice to have but given the precedent of none that has been set i’m not going to hold my breath. I also think that we’re only going to get a new architecture set if it comes with a DLC so it can be monetized, which means considering what new civs could get this new architecture set along with old civs getting it too is something to keep in mind (another reason why Barbarian/Nomad and E. Asian have higher chance of happening) and tbh I’d say unique castles for preexisting civs is something i’d like to see first.
Eh, honestly I don’t think civ-unique TCs and Houses are necessary; just leave it at castles and then start doing region-specific Monks and what-not. Beyoond castles i’d rather see some regional variation in unit sprites, or all new architecture sets, before we start messing around with civ-specific other buildings. Any civ-unique buildings other than castles make things too messy and confusing IMO