How about Thesaloniki - Sofia? The Timetime show 2015/2016.
For now it's running, but haven't seen Greek/Bulgarian timetable after Dec.11, so no definite info regarding 2016/2017
I found an new connection: http://p.bdz.bg/v/a/varnaburgassofia-moskva-sofia-2503-6017.pdf
oh its just for 2016. Hopefully the operate 2017 to.
For those Russian carriages global price is used, so you'll have to pay 92 euro for Sofia-Bratislava 2/3, but again who knows what will happen to those through carriages (imho, here chance that they will be canceled is much higher than Thess.-Sofia)