One of the 3 aliens stronger than x
Tak from Invader Zim
I think this might happen anyway since this wiki has a page on his universe but none the less I'd still love to see Rex Salazar to make an appearance in this series
Defiantly Generator Rex and Tetrax Shard!
Generator Rex and Dark Danny as well as some of the canon transformations we never saw in any of the shows or movies but are still canon (Ex. Shellhead, Rocks, Squidstrictor).
Here's a random answer that I don't think anyone has thought of before, The Spectacular Spider-Man.
If I had to choose one, Walker for DP, and for B10 I'd say Reiny (Think I'm spelling it right).
Also, Marvel's Thor appearance would be cool.
I say any Ben 10 alien but with overkill on it
I want to say that how about we can see more ben's from alternate universes and one of the strongest ben who watch is way much stronger and powerful than any other ben universe or any other universe