Except Disney most likely owns the rights to Star Vs. and I don't think they'll let go of those rights.

Even thought atm they're done with the series?

Yep. Just because Star Vs. is done, doesn't mean that Disney doesn't still own the rights to it.

Not to mention, Disney is EXTREMELY hardcore about their copyrights - so much so that they have quite literally gotten laws made in Congress just so they can keep the rights of 90-year-old cartoons. I guarantee you they’re putting a ton of resources into getting Spider-Man all to themselves, and back on topic, they’ll never let the rights to any of their properties - Star vs. included - even a teeny bit out of their sight. The best we can hope for is a Disney+ continuation since the series is leaving Hulu and going there, and also because Disney severed their connections with Netflix entirely to make Disney+ (something Nickelodeon did NOT do, hence why we got the Invader Zim and Rocko specials that you’re most likely referring to). My guess is, if the Phineas and Ferb movie is successful, they’ll probably expand and make movies for their other major cartoon properties, and even then we’re probably going to get Kim Possible and Gravity Falls movies beforehand just because they’re hotter commodities.