Some things I would like to see in this sequel.
1) More guest characters Perhaps even making the game play like Super Mario Crossover, giving guest characters a special move from their series.
2) Guest characters with 16bit models and 3D models. Yeah, I know this is more work, but it would add so much fun and value to the game and experience.
3) New items retro-ified and new items in general. Just because this is a Super Mario Maker, doesn't mean they couldn't add a brand new power up or 2.