Do something with time travel. We know it's real in the world of SvE, might as well do some epic feature-length storytelling with it.
I'd love to see something along the lines of Mark's recent comic, but it's actually too dark for a cartoon, if you think about it: Marco has to choose who to deny the right to exist. So maybe something with just one kid. Taking place later in the show, when Marco and Jackie already broke up, and Marco is getting on with Star. Their daughter goes back in time to meet her parents because they screwed something up in the future, and she is trying to fix it. Or better yet, two children, a brother and a sister, with a sort of genderbent Starco dynamic.
I especially like the idea of a war between Earth and Mewni going on in the future, so the kids go back in time to prevent it. Being descendants of both worlds kinda makes it worse for them. They don't reveal their identities to Marco and Star at first, but are forced to tell the truth eventually, when one of them accidentally drags one of their younger parents back to their dark future, so both pairs are split up across two timezones, and each character has to work with their future child/past parent to reunite with the others. When they all return to the present and fix everything, the future children fade away, because with the bad future prevented there is no reason for them to have gone back in time in the first place, so it's like they never even were here. Star is like "no, don't go", but Marco tells her "come on, Star, we'll meet them again in a few years anyway. Y'know, when you give birth to them in the first place"
Hell, maybe even tie it up with that crazy "Janna is actually Star and Marco's daughter from the future" idea. It gets less likely with every episode, but at least it could mean that she actually had means to meet John Keats, on whom she supposedly has a "crush" on.