To give India battle elephants would mean completely restructuring the civ. They would need nerfs to economy. And might become similar to some of the battle Elephant civs. It is not in a bad spot right now competitively, so drastic changes are not required.
Though to address the issue of not having a good Elephant unit for the theme of the civ. Reworking the Elephant Archer to make it more useful and give it a versatile role will be better.
I made a suggestion in another topic-
This is a selective buff, which makes it better against some units but still be countered by some other units. And doesn’t overlap with role of hand cannoneers or cav archers. I don’t think this change will make the civ op. Only the unit will be more useful…