Another possible newcomer for STN could be Montenegro Airlines,showing at least on the ACL capacity declaration for Summer`18.
No official announcement yet.
Also Maleth-Aero although not new for STN are showing with slots for BCN/FRA which could be "holding" destinations.
Possibly more unlikely to happen & with the current political situation too is Nordwind who have Summer slots for a ZIA(Zhukovsky) -STN service.