We're thinking about doing something like this, probably with the easy mode (similar to your suggestion) only the final boss would have a loot table & relic, so you could still get decent stuff it would just take a lot longer. Needs a bit of thought for balancing though, we'd want to try and not destroy the queue for normal mode dungeons.
Ascended dungeons would probably not get a single player easy mode though, I think we still need the best rewards in the game to be on the hardest group content, seeing as the game is supposed to be multiplayer