British was not the first to unite the subcontinent. Mauryan empire did it first in the recorded history. Mughals did it too. Marathas were almost able to achieve the same when British finally defeated them. Apart from political boundaries, culturally there has always been a strong bond. If we meet in western countries, there is an instant connect between us irrespective of political boundaries here. People from Nepal to Tamil Nadu have similar mythologies surrounding Lord Ram. So, it was not an unnatural union. Although, I am happy that Indians found a better glue to stick together due to common hatred of the British rule. We now realize that united we stand, divided we fall. So we will never be divided again, and solve our problems without breaking apart.Or the alternate reality where British did not create this unnatural union of vastly different peoples.
Forget any model with Pakistan, India itself is far more diverse than entire Europe. The British turned a continent into a country and it's not sustainable in the long run. Forget about adding other countries.
I am not talking about adding any country. I already mentioned that there is no going back. I am saying we can aim and hope for a France-Germany type friendship model in the future.
Kashmir is a topic which needs a lot more explanation so will do in a different post.