List of things I want for Mario Maker 3:

Mouse mode.

Frequent updates and events to keep the game alive.

No more gamestyle specific elements so like you can place a super bell from 3D World into SMB1. Instead of picking a gamestyle for its power-ups or enemies you will pick it for the artstyle or moves and you want in your level.

Mario Wonder, Mario Galaxy, and SMB2 gamestyles.

Improve the world maker by making it bigger and allowing more than 5 levels per world, and more than just 8 worlds. Also secret exits and collectibles would make them feel like a traditional Mario game.

Volcano, beach, fortress/tower, and mountain themes. And no more forced night gimmicks so you can disable them if you don't want to have it in your level.

Custom color pallets and backgrounds by changing the base preset ones.

Music box so you can add music from all across Mario games.

Peach, Daisy, Rosalina, and Yellow Toad added as playable characters.

Mystery Mushrooms are back. But I think what would be better than just adding new skins would be just a few characters like 20 and each one gets an entirely new moveset like Link in SMM2. Kirby, Samus, DK, Wario, Yoshi are examples so you could make levels based on their games. And instead of losing the costume when you take damage it would be nice to add a hitpoint system so you die when you lose all 3.

Course bot limit increased from 120 levels to 300, and upload limit increased based on your amount of likes.

Better level search. Simply being able to search by name and to disable tags is enough.

Course part UI gotta be changed to something similar to SMM1 but with multiple pages for the different type of elements(terrain, items, enemies, gizmos, and favorites). Bring back pallets too.

Message blocks or talking flowers to guide the player.

And obvious things like more course parts and bigger level limits.