I don't think that we can pretend it won't happen. If not in KH3, then in a future game. Cruella de Vil is the highest profile Disney villain not to make an appearance in the series. Those of you who cite the KH1 side quest as reason why, there are still ways. A Pinocchoi based world was created for KH3D, even though Jiminy Cricket and other characters have appeared in the series previously. They could obviously recanonize the side quest as noncanon, or have it as another time travel world or some other explanation.

 

So, how would combat work? Would the player character turn into a dog for the world, or something (like for The Pride Lands)?

Nah, I imagine Cruella's selfish heart creating a heartless or turning herself into a heartless like Clayton and Stealth Sneak and you'll just have to fight her. It will just be like Deep Jungle, but puppies and a city instead of gorillas and a jungle.

i think it would be odd beating up some woman who just crazy and nothing more i understand she could be taken by darkness and all but i cant see myself brutally beating cruella Devil

All I know is I want to fight Cruella de Vil. She creeped the hell out of me when I was a kid. I want to wack her with a keyblade.

 

But that probably will never happen

Well, if you're a dog then it would be quite cool; fighting a titan sized boss. . . and that way you wouldn't feel so bad about beating up a crazy old woman.

tutti frutti. I'm kind of having a hard time doing an actual world for the 101 Dalmations. I can see if Square brought back the side quest of looking for the puppies again. Maybe Square could make it similar to the Pride Lands world, where the Sora is among the Dalmations looking for the puppies and helping them stop Cruella De Vil. But if a world is done for 101 Dalmations in a future KH game, it would be something to look forward to in terms of what to expect.