If the countries of Afghanistan and Pakistan were to unite, I hope they honor their pre Islamic Greco-Buddhist heritage and call this union by the name of Gandhara. And, of course, the national language would have to be Persian, since most Afghans (including the Pashtuns) speak a dialect of it (Dari) and Urdu (the national language of Pakistan) is a milieu of 60% Persian and 40% Sanskrit derivative.
Urdu is a Turkish word that means (military) Garrison; as the language was developed in the garrisons of Mughals where the Turkic-Afghan contingent mixed with their Indian counterparts to eventually form a language called ''zaban'e urdu'' (language of the garrison) or simply Urdu. The language was further encouraged by the British and it was mostly under their tutelage that the Muslim elites of India switched from Persian to Urdu--which then became the national language of Pakistan post-partition, although the gov't language has always been English. In 1947, very few Pakistanis spoke Urdu, but today almost everyone does. But the mother of Urdu is of course Persian (Farsi) as is obvious by the fact that the vernacular Urdu often has lots of words from Hindi, but the classical language emphasizes Persian (including Arabic based on Persian grammer and phonetics).
Gandhara was a civilization that spawned out of Greek infleunce on Bactria (Afghanistan) and India (Pakistan's Indus river valley that later came to designate all of South Asia under the Arabs and persianized Turks). It gave birth to Hellenistic Buddhist art and also the Mahayana Buddhism in Taxila (modern province of Punjab) and Peshawar (in North West Frontier Provnice) of Pakistan. The second largest city of Afghanistan, Qandhar, is a variation on the Sanskrit word Gandhar(a).
Pakistan is a cultural mileu of two ancient and two modern civilizations: the ancient are Indus valley, and the Greeco-Buddhist Gandhara civilization. The modern are the Persian Islamic influence, especially under the Mughals, and the Anglo-Indian British civilization. Unfortunately, since the 80s emphasis upon Islamization corresponding to the Soviet invasion and the frontline status of the state in this cold war drama, the people have forgotten how much of their country is a product of Western influence, epecially those of the British and the Greek.
I hope that one day the Pak-Afghan people could proudly claim themself as the eastern frontier of the Western civilization; Beyond the Khybar Pass of the HinduKush (Caucasus Indicus) and at the threshold of the mighty river Indus, where Alexander's armies were finally thwarted but the influence of the Greeks was assimilated and gave birth to a civilization that although influenced by Buddhism from further east (Maghda or modern day states of Bihar, Bengal, and Orissa in India--whose historic name is Bharta while the historic "India" was of course the contemporary state of Pakistan), nevertheless, molded it with a very humanistic culture that resisted the Brahminization and the spread of caste system to the north west until its demise with the onslaught of the Turks and Mongols from the steppes of Central Asia, following a long tract preceded by the Huns, Scythians, and most famously the Aryans circa 2500-500 BCE.