A couple I remember from the 80's:
Transformers - Cybertron is restored, the Headmasters are sent to Nebulos to destroy the last of the Hive's machines, but the Decepticons are left roaming in spacing, vowing to plunder other planets and create a world greater than Cybertron
Sledge Hammer - show was cancelled, first season ended with everyone being destroyed by a nuke. Show got picked up again, with the beginning of the 2nd season completely ignoring the ending to the first. Given the show's nature, I'm assuming this was done tongue in cheek.