Now that Leoreth is basically done with unlinking civs and slots, it is probably a good idea to revisit potential additional civs, some might be partial, and some might be playable:
Already existing in some form:
Mamlukes, currently respawn, but don't have stuff fleshed out as a playable civilization
Peru, also respawns, but also without flavour, make into a Peru-Bolivia Confederation?
Turkic, different incarnations of Central Asia with more flavour?
Commonwealths
Boers/South Africa, a popular request, fills a good bit of dead space
Australia, also population, fills a giant amount of dead space
Indochina
Vietnam, Cambodia hasn't been a major player since the Khmer, but Vietnam could be a new interesting player, given the repeated Chinese attempts to take it over, plus interactions with the French.
Burma, fills a good chunk of space that Siam and Vietnam don't really fill. Taungoo tried to expand and conquer the rest of Indochina so that could be an interesting point, then later friction for European colonized India.
Indonesia
Indonesia is quite large and historically quite culturally varied, just one civilization seems strange, maybe split it into Javanese (Majapahit) and Sumatra (Sri Vijaya) civs? Should there be one for Malaya and Borneo? This was an important area for trade, religion, and conflict
Middle East and North Africa
Maghreb seems too big for just Morocco, need at least an Algeria, maybe also a Libya and even a Tunisia?
South Arabia, there should probably be something for Yemen and Oman, they had holdings south along East Africa, and traded with the Indian subcontinent,
Iraq and Syria? (Babylon and Phoenicia?)
Europe
Should Scandinavia be split into a Denmark/Norway and a Sweden? Basically one to go west, and one to go east?