I will try to make a well-designed Super Mario World hack; what are your suggestions?

I have decided to play around a bit with "Lunar Magic" and create my own stages, and possibly make a ROM out of all those stages some time.
However, contrary to a lot of other hacks, I am not aiming to make the game "extremely difficult" or anything like that - sure, the later stages will of course be fairly challenging (and sometimes infuriating), but they are going to be challenging in a way so that all losses are the player's own fault.
My goal, instead, is to make the game feel as authentic as possible, so that it could have worked as an actual official Mario game.
Now I am wondering, what would you expect from a game like that?
Is there anything that should be included, ot left out?

My experience with Mario games is that they typically start out on a fairly straightforward plain/grassland stage, usually with Goombas, Koopas and pipes, and also question blocks and item blocks here and there.
I also know that a lot of people, including myself, hate the "invisible block" thing, where you hit an invisible block right above a hole and get an unfair death, so I am not going to include that.

I might post a few images of some of my first stages, and let you decide if you think they would work.