My 2 cents, That's for the people of KPK to decide.
Of course under current circumstances no one would leave the Pakistani federation for Afghanistan and I as an Afghan wouldn't want that either. It's not in the best interest of the people of KPK. Even though Pakistan is not perfect, it's a better alternative to current Afghanistan.
In the long run, these modern borders will change. There are simply too many ethnicities and languages in Afghanistan and Pakistan. One solution is to make Farsi or Urdu as the national language and unify all these people into a state. But this comes at the expense of sacrifying local languages and identity for the all encompassing national identity. This idea is not acceptable to me as a Pashtun. I want my lanaguage and unique identity to be preserved. In both Pakistan and Afghanistan, my language has not flourished because it has take backseat to Farsi and Urdu and now rven English. So in the interests of my language, I prefer a unified Pashtun state with losely defined borders which is part of a bigger union comprising of Punjabis, Sindhis, Tajiks, Kashmiris and Balochis. I don't classify this as racism or anti Islam but fairly natural.
The Pashtuns who live in Karachi can stay there as long as they respect the local laws and culture and might even be assimilated into Sindhi culture one day.