Interesting indeed. According to Wikipedia they are also starting Nis to
SXF; Ryanair's first routes in Serbia.
The Balkans are a big gap for Ryanair, they just fly to some Croatian beach destinations (Rijeka, Zadar, Pula) and and a few routes to odd airports (Osijek, Podgorica, now Nis).
Not a single capital like Ljubljana, Zagreb, Sarajevo or Belgrade. IMO places like
SJJ or
BEG would be ideal for Ryanair in order to stimulate city-break traffic like they have done in the past in places like
VNO,
BTS or
RIX. No one compares to Ryanair to boost traffic in that way.