Prevent the Yuezhi invasion (maybe through the Qin Dynasty of China not collapsing after Qin Shi Huang's death, thus preventing the rise of Modu Chanyu, thus improving the Yuezhi's territorial situation in Gansu), and keep Persia under Greek rule (Rome and Parthia don't partition the Seleucids), and the Greco-Bactrians could have some breathing space to consolidate. Maybe they could leave a residual ethnic minority for the future, similar to the Tajiks.