The Sogdians were still using some Greek units of measurement in the 7th century AD, and the Bactrian language (which is an Iranian one) was written in a version of the Greek alphabet until the Islamic conquest of the region. Hellenistic art was a major influence on art across western Central Asia and Northern India, and a major contributor to Buddhist imagery. As people have said before Central Asia was pretty Hellenised, on a fairly large scale considering it's thousands of miles away from Greece proper. The issue for your idea is it remaining that way, as opposed to the Greek influences simply being subsumed into the general cultural sphere and more obvious influences taking over.
I don't know how you can add to this in the long term; the Hellenic populations in Central Asia and Northern India created fusional cultures involving the pre-existing peoples there, and it was only ever going to be 'fusional' so long as there were large populations of people who primarily identified as 'Hellene'; long term, cultural fusion is going to result in part subsuming of at least one of the cultural identities involved, especially given that there were no new Hellenic colonists in places like Bactria after about 130 BC.
The Seleucids were the most important state when it came to the settlement of Hellenes in the Middle East and further east, and are almost solely responsible for the large number of Hellenic populations in Bactria, along with its early economic redevelopment (not that it wasn't rich before but, you know, war). Anything that 'extends' Hellenic influence to become more lasting almost certainly involves the Seleucids, because once the Seleucid Empire is pushed into its Syrian heartland I don't see a plausible scenario where a Greek speaking Empire re-emerges and reconquers the Parthian state. Any PoD would probably involve the Parthians or Seleucids.
Also, the notion that the Parthians were anti-Greek is an old fashioned one that I see a lot and isn't the general modern interpretation of them- there were several Parthian kings who epitheted themselves 'Philhellenos' for goodness' sake. The real unravelling of Hellenic presences in the Middle East comes with the Sassanids, and even they still put up bilingual inscriptions with a Greek version for a while.