We can expect Iran to get more involved by aiding those fighting the Talibs; especially the Shia Hazaras. In 1998 after they took Mazar i Sharif the Talibs killed a few diplomats and Iran thretened to invade. Not many are aware but in 1863 British troops landed on Kharg Island as a warning to Persia which was involved in border clashes with the Afghans.
As for Tajikstan, Turkmenistan and Uzbekistan; if the Americans really leave and the Talibs gain an upper hand; we can expect these countries to supply arms to their ethnic brethren in Afghanistan but nothing more than that. Tajikistan in the past played a vital role as a supply hub in the 1990's for the Tajik dominated Northern Alliance.
Times have changed. Iran has began forging relations with Taliban. The Shia-Sunni argument is increasingly becoming meaningless for Taliban as it pursues common interest with most Afghan neighbors (predominantly China, Iran, & Central Asian Republics under the umbrella of Russia). Taliban's vehement anti-Shia agenda is on a slow creep nowdays.
The fact is, all of the Afghan neighbors have began forging relations with Afghan Taliban. There today is a possibility that Russia might become Taliban's main supporter. The American generals in Afghanistan have already mentioned Russia providing weapons to Taliban. Assuming that Haqqani network does enjoy all forms of support from Pakistan & that Pakistan does give them up under the American pressure they would simply fall into Putin's lap - which would be lot worse for the US.
Turkmenistan, Uzbekistan & Tajikistan are highly influenced by Kremlin. There is a possibility that groups from these ethnicity would join Afghan Taliban in large numbers in coming days. In Pakistan when the military operations were conducted against the militants many of them turned out to Uzbek.
America has to come to terms with these realities:
* For as long as US is in Afghanistan the Taliban would continue to fight. They are very relentless.
* The Afghan Army is on the verge of collapse.
* Afghan government with its dubious legitimacy, high corruption rate & incompetent administrative ability could follow soon. It exists only because of the US presence, not because of any popular support.
Additionally, you can not expect help from Pakistan while:
a) Empowering India in Afghan affairs
b) Ignoring the Indian support for the TTP
c) Ignoring the NDS & Raw sponsored terrorism inside Pakistan through Afghanistan.