I think the chance for that is over, several conflicts have solidified separate national identities in all three countries of the former British Raj.
If an Indian conquest of Pakistan didn't turn into an Indo-Pakistani nuclear war, China would get involved to protect the independence of one of its closest allies.
The best historical chances for a united India would be a scenario where Britain treats the veterans of the Indian National Army* more harshly, and there's a war of Independence to kick out the British.
*The Indian National Army was a Japanese-backed force made up of British POWs who switched sides and fought for the idea of a united, secular India during WW2. There was a famous trial at the Red Fort in New Delhi of three INA officers, one Muslim, one Hindu, and one Sikh, that sparked massive outrage against the British and protests where Congress and the Muslim League cooperated.
I think that Subhas Chandra Bose, the INA's leader, could have served as an Indian Tito or von Bismarck type figure who lead a unified Indian independence leader.
He died in a plane crash on Taiwan in 1945 when he was traveling to Manchuria to try and get Soviet support for Indian independence