If in near future South Asia get integrated on the model of the European Union (If Happened) then Kashmir or Khalistan or Similar issues could get solved??
It is too immature to assume that European Union superstate model is a success there have been emerging cases of numerous conflicts of interests on issues such as Trade, welfare, Immigration and policies on foreign engagements and military. BREXIT and the emergence of a populist Right have shown how the EU experiment is doomed.
Coming to the question of South the Asian Union, practically it’s just not possible. The EU emerged as a condemnation to the Hundred year of the feud between European powers that ended up taking lives of a hundred million people. We in South Asia are yet to witness such competition and an all out warfare. The common denominator between the European countries was Judeo-Christian culture and separation of church and state. In South Asia, few nations which are synthetic (Pakistan, Bangladesh) have made their identity around religion which prevents any kind of secularisation that can cause breakdown of nationalistic identities. The issue of Kashmir and Khalistan is a mere fringe separatist movements that are supported and funded by hostile states of India, most of these insurgencies or separatist movements emerged as a result of political negligence or opportunism. Our hostile neighbours just used it as an opportunity to fracture India in order to gain more geopolitical space. If one looks at instances of separatist insurgencies around the world such as those in Chechanya , Palestine and China then one can see they emerged as a result of change in demography and suppression of locals by the mainland population. India’s case is pretty much different.