Do you ever play as Maya? Or Aztec/Inca? 90% of the time these civs either die without contacting another civ or die to the first civ they come into contact with.
Comparing IVC to the Incas,Mayas, Aztecs is a bit ridiculous. Mexico City today is home millions of people-how many people live in Moenjo Daro? Native American languages are spoken by millions as first language in Ecuador, Peru, Southern Mexico, Central America; how many people speak -----? There are 50 million people that identify themselves as Native Americans in Latin America while the vast majority identify themselves as Mestizo. Also the Native American civilizations have a specific purpose, they not only represent Native American culture but a massive event in human history which was the conquest of the America's.
Harappans can be much more interesting than those three because of its location in the center of Eurasia with potential for both eastward and westward interactions. They can migrate to the Ganges valley and defeat/prevent the Vedic India from spawning. The can expand toward the Persian Gulf/Transoxiana to compete with the Persians.
I have no qualms of civilizations acting ahistorically but you want to add a civilization based on ahistorical situations based on your imagination? Well you know what lets add Israel because they are in a perfect geopolitical situation to conquer the whole of Near East or Sythians because they could have easily conqeured all of Eurasia.
And you're wrong about the lack of historical interactions. Harappans/IVC traded extensively with Mesopotamia and Egypt. Artifacts originated in those areas (as evidenced by their style and make) have been unearthed in IVC ruins.
Im not wrong, as I said "real" interaction (and i did know about the artifacts). And by interaction, I mean how technology was widely transferred between the Egyptians, Mesopotamians and later the Greeks. How the Egyptians fought the Neo Babylonian Empire, how the Mesopotamians fought the Medians and the Acheaminds for land. At the end of the day, the fact remains that the Harappans are supposed to be there for the first few turns and their only real purpose is to open border with Babylon for 5 turns then get destroyed by marshes or barbarians never to be seen again or heard from again.