idk if anyone has mentioned this stuff or not, so...
While playing 1.4, I've noticed some things about the physics or some of the power up stuff. For instance, I'm used to being able to glide slowly with the Super Leaf or Tanooki Suit (either holding down the jump button or repeatedly pressing it). However, it doesn't do that at all in 1.4, which I think really breaks how some of the Mario games have always worked.
The second thing to improve the game would be an easier way to add music to the Starman, possibly even Mega Mushroom. I know there's different ways to add music to the Starman through scripts and events, but I feel like, though default, the Starman and Mega Mushroom should have songs assigned to them for easier use. The music could also be able to be modified in the sound settings on the World Map editing mode.
as a side note, apparently if you have too many gfx in a level, some of the custom ones you put in won't show, like Mario textures, blocks, etc. pretty sure the gfx expansion thing would be able to fix this, though some people may not have all the correct programs for image editing, or perhaps their computer isn't able to run those programs. again, just a little side thing I'm adding in.