Cars is already coming out with Planes, so what spin-off would you like to see?
Reject-Bots: The Spin-off
Ok, and a few others:
Remy Can Cook!
The Incredibles: The Series (it would make for a great episodic series, and they would fight new bad guys each time)
Cars should really get it’s own show
Yup, these are all my ideas for Pixar spin-offs. 
I second the motion for those series ideas, JSWeC! Especially with Cars. After all, it’s hard for me to imagine what would happen in a Cars 3, but a show could keep up with the characters in Radiator Springs and all around the vehicular universe! If not that, maybe Planes could… either way, there’s a good chance they could actually do a show!
A reboot of Buzz Lightyear of Star Command with an actual plotline or a tone similar to Batman: The Animated Series would be nice. But I guess it won’t happen… 
How about a Monsters University spin-off with Oozma Kappa as the main characters, interacting with all the other students, and maybe some occasional appearances by Mike and Sulley, and possibly even Randall?
This was mainly an excuse to have the Greek Life characters more in the spotlight… 
JSWeC and TSS: I second both ideas 100%! 
A spin-off about the Tank Gang, perhaps?
The Adventures of Mr. Incredible with Frozone and Mr. Skipperdoo.
^Um…wasn’t that already covered in the bonus features?
Y’know…it just might have possibly been a joke.
All the same, I’d dig a web series like that.