One of the many reasons Starcraft 2, while being older than Diablo 3, is still able to federate players nowadays, is because developers allowed mods through the Arcade.
It was then possible for everyone to design mods, models, missions, campaigns, thus enrich the content by a hundredfold.
I think an Arcade mod would benefit to Diablo 3, and bring to players all the new things they are expecting to fight boredom.
It is clear that developers alone do not have the manpower to bring new stuff in the game at a fast pace, so they should crowdsource the work via an Arcade mod for Diablo 3, which is basically a mod store (like an app store but for mods)