even though they wouldn't use anything from the cartoon to the movie at least it would be considered highly unlikely i would always love to have seen with todays CGI advancements

The Flip Side edpisode made fully into a movie, always seemed like a cool idea.
Citizen Ghost - where the originals battle there ecto-plasmic counter parts
Or main villians like SamHain, Boogeyman or the Sandman make an appearence. But there all just fantasy wishes rather then anything ever coming from that.

Other then that i have always wanted to see the guys enter the ghost world thats something in the movie universe i always wanted to see happen.

Personally I think the episode with Cthulhu would be a good building block for the 3rd movie, If not I'd pick Samhaim or Ragnarok.

I would love to see a live action Flip Side with apocolyptic Boo York

The Boogeyman will be cool,my little brother will not go to see the movie if he stay on it. Hee,heee. /D

Sam Hain would be my choice. There are some other cool monsters from the cartoon that would've been cool to see in live-action form, but I think using one that is powerful enough to take over the world would be the best choice for a movie.

wow, this a dangerous question to ask a bunch of hardcore fans hahaha

Samhain
The Boogeyman
Killerwatt
Cathulu
The Grundel
The People Busters ( that would be cool!)
The Vathacks ( but we all know that would just be a rip off of the cenobites from the hellraiser series)

or maybe with thier real good writing team, they might be able to come up a completely original villain just for the new movie