The only meaningful sanction will be bar Bulgaria from the qualifiers for the 2024 competition (As Ken said, not much point barring them from 2020 as they're out already out), but there's no way that will happen.
Other than when the Liverpool fans caused the death of 32 Italians in Heysel, UEFA have form minor sanctions.
Once the "thorough investigation" by EUFA has been completed, I suspect Bulgaria's next home game will be behind closed doors and they'll get fined a few thousand Euros.
Greg Clarke, in an interview, said (paraphrasing) they appeared to be making political gestures. For Funks Sake, it was obvious to everybody they were performing Nazi salutes, so why hell didn't he just say so?