What sorts of PODs would benefit the Serbs in the long run? Stefan Dusan having a better military career against the infidel as well as a far better successor?
It would help a lot if Stefan Dusan doesn't suddenly die for no apparent reason.
For example, he lives until 1370, and when the time comes to fight the Ottomans, he treats them as a dangerous enemy deserving every possible precaution (unlike his replacements who got slaughtered because they couldn't even be bothered to set up guards for the encampment). After that, a coalition of Balkan Christian rulers helps drive the Ottomans back to Anatolia.
(Dusan also realizes his son is becoming much more passive and interested in monasticism than is entirely healthy, so he either, well, beats some sense into him, or designates a more worthy heir.)
With decentralization averted and the biggest threat to Serbian independence and prosperity resigned to Anatolia, the Serbian-Byzantine Empire continues to grow and prosper in an arrangement not entirely unlike that of the Polish-Lithuanian Union, consolidating and expanding its sphere of influence over the southern Orthodox world. Centuries later, this union might break up into several pieces, but even after such a break-up Serbia could easily be not only somewhat larger than in OTL, but a prosperous nation of over 15 million people (by 2000).
Most importantly, Serbs will also have the opportunity to be incredibly smug to any former subject nation on the internet.