I bet most of this will never happen so don't get your slopes up. Anyways
More level themes (desert, snow, athletic, beach, etc.) and alternate background choices. Maybe also a large decoration object like a tree or something, but that's not really necessary.
Vertical levels and wrap-around like that one world 8 level in NSMBDS
Checkpoints
More kinds of dynamic platforms to mess with (snake blocks, rotating platforms, airship bolts, tilting platforms, swinging platforms, etc.) naturally all of these could be attached to line guides
Standalone water and lava blocks, as well as quicksand + water/sand/lava geysers
Ice flower would probably be a pretty easy powerup to implement
Throw blocks and SMB2 mushroom blocks
Yoshi berries (decorative in SMB/SMB3) and SMB2 vegetables
Bubbles and parachutes that can contain enemies and objects
! switches and corresponding blocks
Those giant skewers, with functionality similar to the burners (shake to reverse its timing, can be rotated)
Wind with adjustable timing and strength
Dark levels and light blocks
? Ball from SMB3; drag over an enemy and it will drop this upon death for boss-death goals; if the enemy falls off a cliff, just give it to Mario automatically
Line-guide chainsaws and ropes (height/direction adjustable)
More specialized boss options (reznor, boom boom, koopalings)
More level themes (desert, snow, athletic, beach, etc.) and alternate background choices. Maybe also a large decoration object like a tree or something, but that's not really necessary.
Vertical levels and wrap-around like that one world 8 level in NSMBDS
Checkpoints
More kinds of dynamic platforms to mess with (snake blocks, rotating platforms, airship bolts, tilting platforms, swinging platforms, etc.) naturally all of these could be attached to line guides
Standalone water and lava blocks, as well as quicksand + water/sand/lava geysers
Ice flower would probably be a pretty easy powerup to implement
Throw blocks and SMB2 mushroom blocks
Yoshi berries (decorative in SMB/SMB3) and SMB2 vegetables
Bubbles and parachutes that can contain enemies and objects
! switches and corresponding blocks
Those giant skewers, with functionality similar to the burners (shake to reverse its timing, can be rotated)
Wind with adjustable timing and strength
Dark levels and light blocks
? Ball from SMB3; drag over an enemy and it will drop this upon death for boss-death goals; if the enemy falls off a cliff, just give it to Mario automatically
Line-guide chainsaws and ropes (height/direction adjustable)
More specialized boss options (reznor, boom boom, koopalings)