Well, the Greater Bulgaria scenario would probably put Bulgaria on a collision course with Greece, because if Bulgaria gets cozy with the UIS then Bulgarian nationalists and right-wingers would probably clamour for Greek Macedonia and Thrace
in addition to Yugoslav Macedonia. This would definitely cause alarm amongst Greeks - since a lot of Greeks remember what happened during the Bulgarian occupation of certain areas of Greece bordering Bulgaria during WW2, where those areas were part of an ethnic cleansing campaign - which was encouraged by the Nazis. So the Macedonia question would probably force Greece to rely more on NATO as well as on their own military because here's a country that's literally trying to rip Greece apart (from the Greek POV)!
Ironically, this could probably make Greece and Turkey closer together in terms of their relations, because the Greater Bulgaria scenario also has designs on parts of Turkish territory, in addition to their attempt to basically force Bulgaria's Turkish minority to become more "Bulgarian" or get deported to Turkey. (Of which the anti-Turkish measures were out in force during the last several years of Communist rule in Bulgaria.) Yes, we know about the enmity between Greeks and Turks, but in this case both Greece and Turkey would probably have an "enemy of my enemy is my friend" mindset if Bulgaria ends up going for Yugoslav Macedonia, because both know they will be next - in TTL, even more so in the case of Turkey because of Volodya's anti-Turkish paranoia. That both Greece and Turkey are members of NATO only helps this impromptu Greco-Turkish alliance in this case. So Greece and Turkey end up becoming unlikely allies if Sofia goes for the Greater Bulgaria scenario, which
might - I stress,
might - lead to closer relations in TTL and a huge diffusion of tensions between the two, maybe even becoming friends. This could also lead to a resolution to the Cyprus question.
So Macedonia, as a part of Bulgaria, could join the UIS. Macedonia joining the UIS could also be aided both those who advocate for a Greater Yugoslavia involving Albania
and those who wish to maintain friendly ties with Belgrade. This tension could be seen in TTL as essentially part of Macedonian life, whose greatest expression is in the design of the
Macedonian alphabet. So, in that case if Belgrade joins the UIS there would definitely be clamours in Skopje to follow the same path (as Serbia is essentially the dominant economy for the former Yugoslavia). Becoming part of Greater Bulgaria would be problematic for Macedonians, for sure, but when you have additional factors in play that would force Macedonia to join the UIS anyway, what choice is there? This, in turn, could lead Greece and Turkey down the road to friendly relations and probably become much healthier for the Balkans as a whole to have two countries become less paranoid about each other. It could probably help lead to major reforms of the Greek economy, society, and culture so that we could avoid the problems Greece is facing now in OTL. It could probably make it all that much easier for Turkey to join the European Union.