Honestly no clue and it probably was different along the Gangetic plain- they could have been majority austro Asiatic, or there might have been a dominant Dravidian element or something else entirely. I was reading a paper yesterday that suggested kosala is a Tibeto-Burman name. As far as I'm concerned all we know is that whoever they were they had retroflex consonants and while that might lead some people to think they were Dravidians on the grounds that Dravidian languages have retroflex consonants, i don't think that's conclusive.
@Shahrasayr Im intrigued but a little skeptical- if the model you're proposing was true, I'd expect there to have been more evidence in the names of rivers- normally incoming immigrants, no matter how much they manage to assimilate the people dont change the names of rivers. Even the tigris is still called essentially the Akkadian name for it in Arabic. But in the Gangetic plain there are approximately zero river names not derived from Sanskrit, unlike say Maharashtra, suggesting that whoever was there before, they were so thoroughly marginalised and cultural collapse had been so through they couldn't even give the names of their rivers, something that even the Native Americans and Aboriginal Australians managed.
Also would you really call it thriving if they didn't form any urban centres?