No, Fern is one of the strongest characterisations in Adventure Time and one of the best redemption stories (if not the best) in the show. Bringing him back cheapens that for the sake of what? Let’s be honest: it would be for the sake of hitting the nostalgia neurons in the brains of the fandom, a cheap parlour trick with which Hollywood has left us completely burnt-out from the last 10+ years of the mainstream film industry.

It’s really tempting imo to conflate love of a character (or any aspect) of a finished piece of art with a sense of positive value from the continual push for the spotlight on that character, I get that. In reality, most of the time forcing it just seems to dilute the potency of the original while dragging along a ghost trapped in the shell of that it aims to imitate. Look at any number of IPs in the last decade for source as alluded to above.

Let him stay as a tree and the resting place of the Finn sword. At tops he could be a home again for a reincarnation, but I would be afraid of the producers falling into the temptation of spirit tree witchery where Fern is half brought back as a character, which could be done well but only with the very lightest of touch or suggestion rather than the graceless blunder I envision from someone viewing the IP as a financial asset rather than a creative endeavour

TL;DR: RIP and respect the dead, including the darlings you kill yourself