Hazards:
Ball n Chains
The Eating Block.
The Falling Ceilings and the Shifting Dirt
Enemies:
Hotheads, Pokey, Ninji, SMB2 Shy Guys, Birdo, Big Boo Boss, Koopalings
Albatoss, Wart/Mamu ,Mouser,Tryclyde,Fryguy,Clawgrip, Mask Gate
Cobrat, Flurrie,Ostros, Panser, Phanto, Snifit
Items:
Key & Key Hole -SMW
Ace Coin
Grass - SMB2
Misc.:
BlueBricks - SMW (The ones you can pick up and kick)
Boss Room
Mini Game Room
Magic Carpet
Mushroom Blocks
Skins:
Super Mario Bros. 2 (Advanced) Skin
Themes:
Dessert
Snow
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As you can see, my focus is Super Mario World and Mario Bros.2
That is because both of those games are my favorite 2D Mario games. I spent a crap ton of hours on both and I love them.
P.S. The Underground theme for SMB2 is Awesome!
It would also be cool If they were able to create 3D Mario game skins, but still keep the 2D formula.
EX.
A Mario 64 skin to change everything to the N64 graphics.
N64 Mario will be able to long jump by running then immediately crouch jumping. the Mario 64 exclusive powerup could be the Wing Cap, Metal Cap, OR the Vanish Cap.
ground : Bomb-omb battlefield
underground: Hazy Maze Cave
underwater: Dire Dire Docks, or Jolly Roger Bay
Ghost House: Big Boo's Haunt
Airship: Rainbow Ride
Castle: Bowser in the Sky