Even then, they just have to figure out how to make the architecture look different. Both, the Muvendhar and Vijayanagara can still be added as base civs. I did made a concept on the Muvendhar while someone else made a concept on the Vijayanagara. So, here’s what they are:
Muvendhar civ concept - Age of Empires IV / IV - Discussion - Age of Empires Forum
Civilization Concept: The Vijayanagara Empire - Age of Empires IV / IV - Discussion - Age of Empires Forum