I'd love to have a dungeon maker in Zelda, especially a 3D Zelda dungeon maker. Would be cool to see everyone's ideas for a dungeon. Preferably not a room tiled placement like it was in Link's Awakening, but more like Super Mario maker where you can place individual puzzle pieces in a room and have multiple rooms Super Mario Maker was very open about that, and I believe they can do the same with Zelda if they put the effort in.
Zelda Classic. There are plenty of halfway decent quests to download online and you can create your own Zelda game from the ground up. Over the years, they've added plenty of options as well. Of course, you are limited to the 2D plane, but for those of us who have been playing since the beginning, Zelda classic has been a long-standing rite of passage.