I don't see why you guys think Fern will get HW so badly, you guys are pessimistic beyond reason.
HW didn't want to date any version of Finn because of her wizard ways, just because Fern is grass won't change that. If she does go for Fern then she would just be a shallow bitch and I feel like the show isn't going for that.
That's without going into how silly it would be for Fern to get involved with romance. His character arc is all abut identity issues, forging his path as a Finn but not THE Finn, the show isn't giving him a girlfriend.
I don't see what's so terrible about giving Finn a new girl since all his current options are shit, Huntress is the least crap but even then she's such a kook I doubt anything long lasting can come from it. Finn getting a new girl would be unsatisfying in a way but the show has been hammering down the realism aspect of his character with romance so that would be pretty realistic. It's at least one step above RS because that show always made it seem like CJ or Margaret was going to win the Mordobowl while in AT the show has been eliminating romantic options so Finn getting a new girl or nobody isn't out of left field.
It's gonna be HW, which would be alright enough, or no one, which would be a bit sad but I could settle with this too. Finn would be able to find someone in the future and my imagination is going to be able to handle one of his romance's a lot fucking better than the show did looking at Flame Princess.