Super Mario Maker is an amazing experience allowing you to manipulate the worlds of Mario games. There are exceptions though and think everyone would agree that they would like to have access to the following:

1. The Koopalings- They have appeared in every Mario game since Super Mario Bros. 3, so the fact they aren't in Mario Maker is just bizarre to me.
2. Super Mario Bros. 2 (USA)- I believe this should have been included just as the case with SMB and SMB3. It is a major title in the Mario games and one of my personal favorites.
3. Super Mario Land 1 & 2- These being included would be amazing as well.
4. Missing enemies- There are quite a few enemies not included that I think should be added in: Boom Boom, Reznor, Angry Sun, Chargin' Chuck, Sumo Bros, Amazing Flying Hammer Bros, Fire Bros, Boomerang Bros, Pokey, Bony Beetle, Twimp,  Spike, Patooie, Big Bertha, Lava Lotus, Spiny Cheep Cheep, Fire Snake, Fire Chomp, Blargg, Blurp, Eerie, Fuzzy, Hothead, Hotfoot, Mechakoopa, Ninji, Porcupuffer, Micro Goomba, Rex, Rip Van Fish, Super Koopa, Swooper, Torpedo Ted, Urchin, and Ice Bros.
5. Super Mario 64- I believe this could be done and would be awesome.
6. Paper Mario- Same with this.
7. More power ups- Frogsuit, Hammer Bros. suit, Tanooki suit, Ice Flower, Mini Mushroom, Mega Mushroom, Penguin suit, Gold Flower, Super Acorn, Double Cherry, Super Bell, Boomerang Flower, Cloud Flower, and P-Wing have not been added but would definitely work well.
8. More scenery/backgrounds- Such as night, desert, forest, snow, sky, bonus/cloud, Mountain/canyon and jungle.
9. More music- there isn't much selection at all currently.
10. More playable characters- Luigi, Toad, Princess Peach, Yoshi, Wario, Waluigi, Bowser, Bowser Jr, the Koopalings and Princess Daisy
11. Secret exits and slopes- Those who played SMB3 and SMW know what I'm referring to.
12. There are quite a few other things we could get to make levels with, Nintendo knows about them and should add them. 

That's about all I can come up with.