Lots and lots and LOTS of anime that are based on light novels and are held back because the light novels haven't been finished yet. Yona of the Dawn springs to mind here as a show I quite enjoyed but felt trailed off at the worst possible time.
Tales of Zestiria the X would have been much better if they'd spent less time on Zestiria material and more time on Berseria material...or not had that one episode which they used to promote Berseria's release. It had a MUCH better ending than the game, but with no prior knowledge of Berseria, it wouldn't have meant much.
Also, Dokidoki Precure could have been SO much better if they'd kept the momentum going after the Cure Ace reveal and not spent so much time on Aguri, who was a poor substitute for Regina.