That would've been neat..
Too bad that, with what we have seen from the show so far, Kelly will most definitely leave Marco before he try asking her out and get together with Tad again after she's done "not being Tad's girlfriend", only that she'd come to appreciate his value even more.
Marco then will realize that he has no luck on Mewni and return to earth, only to find that Jackie and Janna are the proud lesbian couple that Daron teased about, and every girl in school is already taken (even Alfonso, Ferguson, Jeremy, Oscar and Britney have dates).
Marco finally realizes his place in life: in Star and Tom's Castle, as the royal garbage man/ Janitor, watching over the King and Queen's children by day, and watching the King and Queen fucking like bunnies by night.
This is his place in life, it was meant to be from the beginning and always will, so why go against it.
Oh and the Blood Moon and his magical bond with Star transferred themselves to Tom as well. Tom is now the alpha male of the show.
No other show did that, so why not start this totally original, unpredictable and really interesting theme with this show?