1.Only under strong rulers like the Jains, Buddhists or Muslims was the subcontinent ever united. They all ruled with the strength of their faith, which was much stronger than today's "isms". Even the British directly ruled a part of the subcontinent. The other part was leased or ceded to compliant local rulers.
2. No wonder as of 1857, the British sought to decimate the Muslim power base, cultural influence and economic well being. In its place it has gradually brought in a revived Hindulva. Yes, the Brits have been behind the modern phase of Hinduism's rise from the ashes.
3. Communism was tried here early 20th century. In fact the world leaders of Communism had hoped, the subcontinent would be the birth place of a Communist State. Soon they realized they had not fathomed the deep hold of castism/classism over the Hindu society.
4. As unrealistic as it would be to believe the call:" Khalqe Khoda, Takhte Padshah, Hukumate Sipahi" may be heard again to reinstate the Mughal Emperor on the Kohinoor throne, so it would be to imagine the subcontinent would ever re-unite. The Brits played their card well.