My biggest wish is multiple characters, each with their unique abilities. Currently it's just Mario, Luigi, and two Toads playing the same. Give me the SMB2 squad + one or two more characters. If you want two Toads, just make one Toadette. I don't know what her ability would be, but she's a step above Yellow Toad.
Oh and Double Cherry. We always need more Double Cherry.
-A Open Map with various secrets, like Super Mario World
-Could have Worlds and Mini-worlds, just like in Newer Super Mario Bros Wii.
-Three bosses for every normal world ( Fortress, Castle and Koopa Army Vehicle ( Airship, Tank, etc)
-Some new bosses could be included, ex: Petey Piranha, The Big Boo, Mouser, Lakithunder...
-Each world would have a unique theme, but not just a copy and paste of the other games in the series but rather with new themes, like a Space World, Ghost World. The old themes, like the Desert, Forest and Tropical worlds could be reformulated to include some old Mario games references, Ex: Tropical world with Piantas and a level based in Delfino Island, a Desert World with Super Mario Land enemies...
-Each playable character could have unique stats, like Super Mario 3d World, and some characters could be unlockable by doing some missions, like Peach in Super Mario Run
-Some old enemies could return to a 2d Mario game, like the Rexes, Dino Torchs, Shy Guys, Snifits, Chargin'Chucks, Bullies and some forgotten enemies from Super Mario Land 1 & 2 could be reused again.
-A significant improvement in level design
-New rideable characters to include with Yoshi ( Poochy?) And the Super Mario World's Dolphins could return too.
If you see any grammar mistakes, talk to me, please.
Different level aesthetics and more interesting bosses (even if they're restricted to fortresses and the castles have Koopalings as usual) are my highest hopes, aside from that just give me interesting levels and I'm a happy Nex.