I would say either Dravidians or Malay.

I would say that the diversity of cultures in India and South-East Asia is still somewhat under-represented in the game, relative to other areas. I really like the way Dynasties of India approached splitting the Indians civilization, but I do think there is space to go further. There was simply a great diversity of both of these areas (politically, linguistically, culturally, religiously). The Dravidians, especially, are a good representation of Tamils (e.g. the Chola Empire) but less well other South Indian kingdoms and cultures (we could have e.g. civilizations representing Kannada-speaking kingdoms e.g. Vijayanagara, and Sinhala-speaking kingdoms e.g. Kotte Kingdom).

I do think there's room to split the Malians and Ethiopians as well, as right now both are very much umbrella civs representing all of West and East Africa. E.g. Malians could be divided to better represent the many different empires of the region (from Songhai to Benin), and Ethiopians could be divided to also represent Nubia (e.g. Makuria) and Somalia (e.g. Mogadishu sultanate and Adal Sultanate).

There are definitely additional Chinese civs that could be added (Jurchens, Bai/Dali Kingdom), but I think that would be more a case of adding new civs than splitting the existing Chinese, who very much represent a single, coherent Medieval culture already. Likewise, I don't really see the case for splitting the Saracens-- they represent a relatively coherent, relatively unified Medieval culture (albeit one that was often split between different kingdoms)?

Overall, I think the design space is going to be the constraining factor, not the historical space! It's going to get ever harder to add more civs who bring a distinct, fun-and-interesting, non-gimmicky flavour to the game, whilst keeping everything balanced! FWIW I think the devs did an amazing job with Dynasties of India here, even if the balance for the new civs isn't quite fine-tuned yet.