Sorry for the necro, but this is interesting.
Fun fact : Štefánik was at first a keen supporter of the idea of a post-WWI Slovak monarchy with some royal figurehead from abroad as head of state (similar to what the Finns and Lithuanians wanted to pull off). However, in the second half of the war, he abandoned the idea and embraced the Czechoslovak concept.
I like the suggestions given so far, including the rulers of Bulgarian and Bavarian Royal family as rulers of Czechoslovakia. However, the Schwarzenbergs ruling as Czech kings would probably result in their first Charles being called "Charles V" instead of "Charles IV", in order to not be confused with Charles IV. of House Luxembourg, the Czech king from the 14th century.
My suggestion : Maybe the successor state could offer refuge to Karl and Zita Habsburg after they were kicked out of both Austria and Hungary ?
Maybe Karl decides to not tempt fate one last time and retires to Czechoslovakia as its new ruler, instead of trying yet another futile attempt at retaking the Hungarian throne ? Ergo, he doesn't end up on Madeira and maybe doesn't die as soon as in OTL.
Just a wackier idea.
Incidentally, there was a recent Czech allohistorical mockumentary about Czechoslovakia becoming a monarchy in the interwar period. Their choice of monarch was really weird and not all to plausible, though : One of the younger members
of the British royal family (I forgot which).