Yeah this really sounds like an entire new game. I wouldn't want any dev time spent on this within the current game. This could be something like a sequel or something along those lines. Otherwise, as mentioned there are other games filling this niche.
For post-game content though, I'm thinking there just needs to be some harder dungeons, maybe ones where you can more easily come across some of the rarer races. Maybe some additional equipment or higher tier modifiers. These are things mods could add in without too much difficulty.