There are two additions I'd like to see in the Stage Editor that should be relatively simple:
The option to feature 'running off' type edges, rather than always the platform ones. Also, stages ending right in the custom stage bounds, without extra space bellow.
Also, more background options. It was really odd in Brawl how the custom backgrounds were only 2d pictures, and yet they only gave the player 3 options, rather than making every generic 2d background in the game available there at least. They also easily could have added 2d pictures of 3d backgrounds too. I guess part of it is that the piece's "style" was automatically associated with the background - so, 3 themes, 3 backgrounds. But, if the background is just going to be a 2d picture, it seems kind of silly to do that.
Of course, more hazards, unique types of pieces and other things would be welcome too, but those two things seem to be relatively simple and could add a lot more personality to some custom stages, so their absence is annoying.
To me this is about as zoomed out as I would ever want the action to get, and yet this merely represents a normal scenario of two players on opposite sides of final destination (you know what I mean). With 4-players going at it, this would be a totally normal camera perspective on this level.
Isn't that camera more zoomed out than it'd actually be if two players stood at that distance though? But, yeah, it's an obvious issue with the 3ds version, and likely one of the reasons each version has exclusive stages, since the 3ds ones need to be smaller.