I think Zelena and Mal’s returns will be linked–and I think they’ll go back to what Rumple said in 403 about magic changing forms but not being able to be destroyed. (At least I HOPE so.)
With both Zelena and Mal, we saw them turn to magic dust at some point. (And in both cases, they were killed by a blade with high significance to the wielder–we know that objects CAN take on a kind of magic if they’re important enough.) And in both cases the dust was capable of acting AFTER the villain was dead–in ways that seemed to go along with the villain’s desires.
So I think that Rumple may bring back Mal and Regina may realize she can bring back Zelena the same way. Or Rumple will accidentally bring back Zelena doing whatever he does to reactivate Mal.
Either way, I’m excited for Zelena’s return–while Rumple’s killing her made PERFECT sense, she was a fun character to have on the show and I think she balanced Regina out a bit. I’d like to see Regina have a chance to deal with her more.
Some people have a theory elsewhere that Zelena’s ashes might have traveled back in time, which could explain the flashback between her and Marian in 417. To go even further, they also suggest that Zelena might have even disguised herself as Marian, and it was her all along who traveled back to present day with Emma and Hook. Walsh’s furniture shop is also making an appearance in 417 during Robin and Marian’s scenes in NYC. In a way, I could see them seriously doing that, especially in relation to this quote from the interview:
We have a really fun idea that we love for Zelena that we started kicking around toward the end of last year and have slowly been putting into place.
Marian was brought back at the end of the year, so “slowly been putting into place” would fit if she did turn out to really be Zelena. It’s pretty crazy, and it would spring a bunch of other questions, but at the same time I wouldn’t put it past them.