If I were to suggest something almost completely impossible - a custom dungeon tool. You build mobs and bosses together with a set of assets from templates stitch them together and let other players give them a go. Abilities, bosses and mobs could even be collectables from old content. I suspect the data resources required for this alone would make it totally unworkable
Outside of the plane of fantasy; a new class would keep things interesting.
Naturally the usual package of borrowed power to keep things fresh is a must, but I'd prefer it to be slightly more spec orientated rather than class orientated