I spent a couple weeks in Dubrovnik last October and passed through Bosnia on the way up to Split and spent a little time there. Wouldn't mind spending a bit more time in Bosnia, but really hoping Estonia opens up to US citizens now that they're accepting vaccinated tourists (just not from the US currently). Georgia and Ukraine are possibilities too, but I think they're open, vaccine or not. Other hopes are Bulgaria and Romania, two places I would absolutely love to visit.