The way I see it, Pakistani Culture is all three of these :
(a) The Cultures of our different ethnic & linguistic communities.
(b) The overlapping Culture that has been bred due to centuries of continued existence with ethnicities exchanging different cultural practices that were traditionally uniquely associated with another ethnicity. For example most of our older generation is fond of wearing the Karakuli Cap even if its unique to the Pasthun belt, the Peshawari chappal is worn across Pakistan & the koti (vest) that may have its origin in the UP is worn across Pakistan now whether its by the Pasthuns or the Punjabis.
(c) An overarching appreciation of how Islam has deeply influenced our respective cultures & enjoys a special place in our outlook on life & about life & I'm not talking about praying 5 times a day or fasting but rather that standards of propriety, of ethics & morality are indeed a part of a culture & they in turn breed or let die different cultural practices or dispositions. This also leads to a greater appreciation of 'Hum log eik qaum hai kiyun ke hum Musalmaan hain' then otherwise possible in a society with such distinct ethno-linguistic fault-lines !