Wether it was a original project, a sequel, an adaption, or a reboot. There are tons and tons of cancelled movies out there.
I'll list a few I wanted to see myself.
Me and My Shadow : A DreamWorks Animation feature that was said to combine CG animation with hand-drawn animation. Pieces of concept art can be found around the internet along with this pencil test
Superman Lives : A Superman movie directed by Tim Burton and starring Nic Cage would been absolutely insane. This cancelled project is so infamous that there's even a crowd-funded documentary about it
Popeye : God. What I would give away to live in a world where this was made instead of The Emoji Movie.
Gigantic : It definitely would've been interesting to see Disney's take on Jack and the Beanstalk. Even with the songwriters from Frozen on board. To think that we've could've actually gotten this film by now. Now that one in-joke Zootopia isn't gonna make sense to kids.
The Search for Mickey Mouse : Let's thrown in a more obscure one. Mickey Mouse has never had his own feature-length movie and this was supposed to be his first one, and this was supposed to be Disney's 60th film. This movie was House of Mouse: The Movie.
There's more I could list, but I'm too lazy. What cancelled movie did you want to see?