I think in this WI I'd like to imagine more Greek influences in the mainstream population of the Bactrian people. Greek temples, Symposiums, philosophical schools and military tactics. Two things that I think will have the greatest impact to ensuring the survival of Greco-Bactria, is evolving the Greek Hoplite to be more similar to Roman Phalanx, so that their infantry can be better adjusted to the hilly terrain. Adopt Scythian cavalry tactics in order to better defend and communicate with cities within their region. And most importantly, not spread into India to avoid the overreach of their territory, therefore avoiding the bureaucratic issues that came in OTL. The justification behind this could be based on the idea of using Afghanistan terrain to their advantage, not wanting to overstretch and having the empire value internal fortifications instead of expanding land. Even if the Greek population no longer stays as the aristocratic class, a Greek dominated culture might be able to survive until the rise of the Tibetan empire. Since Greco-Bactria had large influences in Buddhist art and depictions, it would be interesting if their diplomacy involved some sort of mutual exchange of resources between the Bactria region and Tibetan region.