The easiest way to make a sequel is to embrace more unique themes with unique items that don't have to connect to the other styles. I know people complained about how 3D World required a full stage wipe and was limited compared to the other 4, but the insane amount of interactions and bugs created from them that existed in both SMM1 and SMM2 is astronomical.
As for what I'd specifically want added to the main 4 styles
- More flexibility with the physics and effects. In SMM2 the night version of courses force things like wind or ice or upside down gravity. Now I love these, but I would prefer that those be a separate option that can be applied to any stage theme day or night. Maybe go crazy and allow players to apply unique physics and conditions to specific parts of a level but not the whole thing.
- Chargin chucks, of all the missing Mario enemies this one has the most potential in Mario Maker. They can quickly break blocks, they can create various projectiles for challenges, and can be used to create an enemy that jumps when Mario does which is currently only in 3D world.
- Increased options for inner connecting levels and having multiple exits. Super Worlds are great for letting people group all their levels together in a unique way, and letting players find curated content from a creator they like. However as Super Worlds were added later on in SMM2, they're still limited by needing all stages to be beatable on their own and stages only having one exit. Enabling players to create multiple exits, and have mechanics like block palaces that open up new paths in old levels would add a lot to Super Worlds and how people design them.
- Add restart from checkpoint. You already make the uploader prove it's possible, why is it not allowed for players to do so? Yes it would hurt the troll communities ability to force people to do anti-softlocks, but it's still a change that would benefit all players.
Ideally, Wonder would just have a level creation mode. There's no need to split these 2 ups other than money for Nintendo.
No, this is an absolutely terrible idea and completely misses what makes Mario Maker so unique. Wonder should not have development time spent on level creation tools, and a level creator in Wonder would be extremely limited compared to what a dedicated Maker game would have. Mainline 2d Mario's need to be able to create unique mechanics/items for stages, and not be worried about how those things interact with each other.