I personally think I'd like better bosses and more exploration. Massive, Super Mario Odyssey-type worlds would be cool. Here's a quote from someone at Screenrant about the rumored Epic Mickey remake:
"In any case, Epic Mickey's universe leaves plenty of potential for a reboot. Imagine an Epic Mickey sandbox platformer akin to Super Mario Odyssey, where Mickey journeys to several worlds within the Wasteland, each with its own theme."
I feel what you said would definitely make a lot of since. Wasteland is of course a Themepark and having quite a bit more stuff in the already pretty decent variety would only make the game even more appealing. I also feel like the ability to go back to certain areas post game with plenty of quests to do (Kinda like what they tried with EM2). The biggest of course would be to definitely keep the game polished. Apparently EM2 has a feature where you can reset a play through without loosing any pins and quests (which is useful especially with again the post game quests), but unfortunately they seemed to rushed the game so badly that they forgot to do this to the Wii version (the version I have, yikes). Pretty much if they make the game more polished than before and just add a whole lot of new stuff I can easily see a big seller!