Maribor has been tried a couple of times briefly, first FR for a season or two at STN then Adria from SEN.
Osijek in Croatia also tried and failed with FR followed by a start-up at SEN that never materialised.
Padubice in Czech maintained FR for a while until Covid but don’t think it’s returned.
Mostar is the big tourist draw in Bosnia, it’s had some recent investment. Apparently Eurowings operated from Germany pre-Covid