~ Vinesauce SMBX Level Contests ~
Vinesauce Vinny hosted SMBX Level Constests in 2013 and 2015. I'm not sure if it there were more, but after then, there hasn't been any more contests. If you search around the web you might find various reasons why from different people.
~ TPC's Weird House ~
In The Princess' Cliche. There is this weird house that completely changes the fun song of the town to a threatening one. The house is full of enemies that can harm you. The only interactive NPC of the house asks you to leave their house. The house takes you nowhere, and it has no purpose except make you wonder why it's there, or if there's any secret behind it. I might be reading too much into it, specially since you can look for secrets with an editor, but I always found it interesting.
~ Nintendo borrowing ideas? ~
Not long after the acquisition of supermariobrothers.org, Super Mario 3D World was announced, and it interestingly had a few similar concepts that SMBX already had before. The only one I can recall right now is the Piranha Plant in the pipe that you can hold, but I think there were more. Later, Nintendo also announced Super Mario Maker. Of course it's not hard to imagine the idea of a Mario level editor videogame, but... Who knows?
~ SMBX Live Level Contest ~
Okay, this one I'm basing only on foggy memories I have, but I recall that the concept of a SMBX Level Contest that would be held in real life was thrown somewhere either in the forums or in the IRC. You would have a limited time to create a finished level, and the levels would be judged in the same day. I'm not sure if this was discussed as a joke or as a legit suggestion, but I remember there were people who seriously criticized the impracticity of holding such contest.