With the custom field options it'd be awesome to see somewhere in the future something like chess.com does with puzzles. That way people can get into learning advanced mechanics in a bit more low pressure environment. But also make goofy 'daily level' for more casual fun.
That and a way to create a series of levels so people can use tetr.io as a way to do puzzle challenges, speedrun challenges similar to super mario maker or just make fun campaigns that could go in a main menu playlist of casual solo play