I really hope that Air Serbia (JU) will consider joining OW at some point. The Etihad Partners debacle is coming to an end with virtually all members bankrupted and EY itself running around in circles. *A is out of the question due to the strong presence of LH Group in the Balkans, while RO would most likely strongly object to JU joining Sky Team (even though JU cooperates extensively with AF and KL). Most importantly, JU is the only airline in the region offering decent (i.e. daily) connections to virtually every capital and secondary city in the Balkans, the latter being a glaring gap in OW coverage. Finally, there is a historical precedent for JU funneling their US traffic through LHR (even though this has lately shifted to CDG and AMS), so BA would likely get some extra feed from BEG's catchment area, which is significant.
One can dream 😊