Nice
Holy shit bro, honestly this is amazing. Some recommendations listed below.
So first off I would make some slanted and or slidey platforms. I think it would add nicely to the platformer feel you are going for. I think you have one slidey piece already so nevermind that. But I would definitely add like a slanted platform where you have to jump from an angle. Maybe not in this version because I understand you don't want to go in deleting full platforms and shit but possibly for another version.
Also, something very very little that takes some time but trust me will spice it up. Make that brown on the plats a little more bright. It will match up more with the actual Mario game.
Another really cool thing could be something like this.
So I see you have some random floating pipes and what not. I was thinking somehow you can make a secondary passageway that goes all the way up to those floaters. Or maybe even lower the floating plats so it doesn't take as long to get up there when there is a pathway up. Also, when making this, make the green pipes so you can fall through to the lower platforms. I think this would be an awesome ass feature to this already amazing course.
That's all for my cnc.
Check out this replay I made in like 5 mins on this map. Its awesome. (the map that is...)