I know this is only a prototype build, but I have a couple suggestions for features that I think should be either in an update or the full release:

Ctrl + Z and Ctrl +Y keys as Undo and Redo keys.

The ability to select multiple objects at once with a selection box.

Different level goals - Flagpole, SMB3 box, pipes and etc.

Enemies from other Mario games.

Visual effects in level, so people can recreate world 3-4 and do other cool stuff.