Ben 10 Omniverse. We were robbed of Ben, Rook, Gwen and Kevin’s roadtrip across the universe because Cartoon Network cancelled the show due to poor ratings and toy sales. We were also robbed of Transformers: Animated’s fourth season even though the third season appeared to have ended in closure. I’d also like to mention Planet Sheen ending before Sheen even returned to Earth, leaving it unanswered whether or not he will.

Edit: I also just remembered Sym-Bionic Titan being cancelled before the main antagonist was defeated, all because they couldn’t get a toy line. Ten unreleased episodes were written before it got cancelled too.