Tom x Marco
incorporate some ladyfying curse from Miss Heinous who still thinks Marco is a girl, just magically disguising as a boy - outrageous. She hits Marco with a powerful bodymorph spell to lock him into his "true" girl form.
Naturally because he's originally a boy it malfunctions, and Marco switches forms for a while over several episodes in a Ranma 1/2 -esque comedic subplot. Tom gets interested in the mysterious princess kicking monster ass with karate, tracks "Marcelina" down to the Diaz residence and starts to court her, unaware that it's actually Marco. Marco is initially reluctant but agrees to the charade due to Star needing him to distract Tom for whatever reason. Cue more Princess Marco stuff with Marco slowly finding himself enjoying the dates more and more, to his horror.
Over time, the fem!Marco side starts to develop a distinctive persona of her own, grappling for control of the body with Marco and flirting shamelessly with Tom whenever she gets the change, sometimes taking control for several days. Shenanigans ensue, and eventually there's a magical clone curse episode where the curse form actually manifests into Marcelina as a being of her own. Initially seeming to be an antagonist, she turns out to just have wanted to exist and needs to be saved from Heinous or whatever.
The slapstick comedy potential would be amazing and you'd solve the ship wars with two Marcos to shove both at Star and Tom. It'd also seem progressive as fuck while still not being all that gay in the end.
TLDR I don't really give a shit how it's done, I just want a genderbending Marco 1/2 storyline with Tom seeing male Marco as a rival for fem Marco's attentions