At this point, Ben and co. have traveled extensively through the multiverse. The Gwen 10 and Ben 10,000 episodes, Paradox's alternate future, Dimensions 12 and 23, the Forge of Creation, and the Generator Rex universe all come to mind. Goodbye and Good Riddance and Race Against Time are entirely in separate universes, and as of So Long and Thanks For All the Smoothies, the entire cast resides in a new universe. Could they actually be thinking of doing a multiverse arc? That would be awesome.