I'm still waiting for the obvious but still deeply satisfying twist that Marco ends up evil and Star has to fight him.
It's been a long time coming, ever since the monster arm thing, and I still believe Toffee know what the fuck was up.
There's a bit I keep bringing up that I think not a lot of people have noticed but, when Marco first sees that magic sandwich, he gets A VISION OF THE MONSTER ARM.
And later on, Toffee offers him that same exact sandwish, I don't think that's a coincidence. And of course he doesn't eat it.
Marco's evil powers are inside him, he just needs an activator of some sort.