Okay, so it has been since 2003 since Lilo and Stitch: The Series, which is based on a Disney Animated Canon franchise, to have only four crossovers (including Fillmore!) to only be confirmed in the same universe with only six shows confirmed.

But what about one of them crosses over with any other Disney Channel Animated Shows, that are still ongoing. Maybe Big City Greens, Amphibia, or The Ghost and Molly McGee. With that, it can have the other shows to become canon to have a whole new multiverse together.

The shows we need to have a crossover with are

Phineas and Ferb
The Weekenders
Teachers Pet
Recess
Fillmore! 
Teamo Supremo
The Replacements
Kick Buttowski
Pepper Ann
House of Mouse hub of any Disney universe.
Gravity Falls
Randy Cunningham: 9th Grade Ninja
Penn Zero: Part-Time Hero
Milo Murphy's Law
Big City Greens
Amphibia
Gravity Falls
The Ghost and Molly McGee