Much as I’d like it to be a nameless nobody, I can’t quite discount the possibility of Rumple being the father either (ofcourse my opinion might be a bit less well-informed than yours, since I haven’t read all the writer-interviews/analysis/spoilers etc.)
For one thing, I don’t think the father is royalty. Because Cora wouldn’t have abandoned a child that gave her an “in” with the royals. Even if she wasn’t initially planning on abandoning her, she certainly doesn’t seem to have made much of an effort to get her back either. Ofcourse it’s possible that we just haven’t seen that side of the story yet… I wonder how Zelena would respond if she found out that she was royalty afterall…
Someone we haven’t met yet/some nameless nobody is certainly a possibility, but would make for less exciting tv. The reason everyone is related to everyone else seems to be: drama!
(Also realism, did you ever take a look at the family trees of the european royal lines? Most royal families from different countries are related to eachother in more ways than one – I’m not surprised that it would be even worse with fairy-tale nobility.).
And in this case the writers have been quite merciless in teasing us with the possibility of Rumple being the father. Not only did they spent an entire episode on the relationship between Cora and Rumple (and already teasing the possibility of a child there), but they also drew the parallel with the shaving as well as with the “scarecrow” comment (Rumple being the scarecrow as he’s the one Zelena got a “brain” from).
Not to mention Zelena is magically powerful and has a disposition for unusual skin-problems. Also, she’s obsessed with Rumple.
Giving her tremendous daddy-issues aside from her already existing mommy-issues, seems very much in line with the show.
But then, if something’s that obvious, in tv-land that USUALLY means it’s not true. And it would’ve been a big thing for Rumple to overlook the existence of his daughter. (I guess even Dark Ones make mistakes).
All in all, I’m a bit stumped. I don’t think any outcome will surprise me at this point. Rumple, Leo, Randompeasant, Santa Claus…. When it turned out she literally fell from the sky and grew up with adoptive parents, it pretty much opened all possibilities back up for me.