Sorry for butting in, but people have multiple identities at different levels. Street, city, caste, ethnicity, language, religion, province, country, race to name a few, nationality merely one of them at national level. And yes, these identity groups exist because they are interest groups at some level. Nobody desires an identity that has no benefit for them.
So supressing a desired identity will always be opposed and will create issues.
As to how administrative provinces in Pakistan be formed, what principle followed etc are extremely tricky situations in India too, and I'm sure in Pakistan. No comments on Pakistani side, but in India we created three new states few years ago quite peacefully but some contentious ones remain open.