I think Indians is nicely balanced civ. Adding elephants would require redesign. Why? It is fun to play as is. And definitely by statistics based on aoestats.io they have a good win-rate.
I think if we try to be historically accurate then the game will be close to Medieval Total War. And that has very less variety with heavy cavalry domination as was during that time in history. With the current civ design we have a lot of variety and yet close to history.