Mario producer Takashi Tezuka recently talked about wanting to expand Mario Maker into other graphical styles.
In an interview, Tezuka noted how "there would be other graphical styles included", but nothing has been decided just yet. He would also like to "include other graphics that aren't Mario", but it will always remain a 2D platformer.
The E3 demo is just a sliver of what Nintendo aim to provide with the final version, with plans to include more enemy/object types, plus possibly a music composition feature that's similar to Mario Paint.
The team are working on internet sharing/Miiverse integration, and Tezuka is mindful of a desire for players to share levels with friends as it's "really the whole point of making levels."
What features/graphical styles are you hoping to see in Mario Maker?