If Milo's Return never happened....
What do you think the sequels would have been about? Clearly the 'sequel' we got was supposed to be the TV show and they had plans to make more Atlantis movies but plans were dropped after Atlantis wasn't a big hit. So they edited the ending of Milo's Return to give the Atlantis story a permanent (albeit very stupid) ending.
I just can't help but wonder what if. I love the movie and these characters, I would've killed to see more :( It's a cool universe with so many characters, at least I think they could've made more stories with them.
(I don't HATE Milo's Return especially in the context that it was a TV show. I was a kid when Atlantis came out and I would've watched the sh!t out of that show. But as for the one and only follow up to the movie, it's awful and I don't consider it canon in the least. And Milo is so damn boring, from being so animated and funny in the movie, he's more vanilla than Fred from Scooby Doo. And it's totally understandable why but Michael J Fox is sorely missed here.)