Here is a couple of suggestions from me:
1. Infinite checkpoints per level. Pretty self explanatory. Being able to have only one checkpoint per level makes me want to rate all SMBX levels that are too long at most 2/10.
2. Solid-on-top slopes. Combine slopes and jump-trough platforms. For instance: stairs in ghost houses or for CastleVania config.
3. World map sections and borders. I always found SMBX world maps being too "open". Being able to limit what player can see will reduce efforts needed for overworld designer to create good world maps. Also it can create more claustrophobic feel to "indoor" maps like caves.
4. More special variables for NPC`s. 8 for normal NPC`s and 17 for containers seem enough. This allow NPC scripters to be more flexible with coding and not to rely on hardcoded variables.
5. Attach layers to player characters. Many gimmicks can be created using this option.
More events and actions for events:
1. Trigger event when level is restarted certain amount of times in a row, like Mercy Mode in NsmbWII or SM3D series, or in DCKR.
2. Action: Destroy Layer. This, when called, destroys all blocks, BGO`s and NPC`s that is on target layer. Their "death/broken" and "Layer Empty" events are also triggered. Good for creating death effects for layer&event driven custom bosses
3. Action: Create effect at X/Y. Again for making events less ugly.
4. Conditional branches for event algorithms. This allows events to do(or don`t) certain stuff depending on specific circumstances (like position of player character, global variable value, random value, powerup(or temporary one) status, timer status, etc.) This will allow to create even more complex events and usem as base for even more bizzare gimmicks.
Hope these things will be good addition for such promising project. And sorry, if something is already suggested.