Cartoon Crossovers

What cartoon crossovers would you most want to see? Free from the restrictions of studio ownership?

What do you mean?

Like for

Example

Scooby Doo meets Gravity Falls

Or

Ben 10 with Phineas and Ferb

PPG 1998 with MLP. It would make sense since Tara Strong already did two characters for those shows.

Would a modern Laff-a-Lympics work? Or is there another kind of show or special that could combine multiple series into a single event? Comedic or serious ideas appreciated.