I love so many things about this game. Story, characters, everything. But sometimes, one just wants to spend 45 minutes killing some monsters after work.
So: a non-campaign game mode where one could assemble a party of four fully-custom characters and get dropped on a random map full of different-each-time combat encounters. A handful of different maps with interesting terrain and features, and an algorithm to populate encounters with interesting mixes of opponents. Have them drop random loot, and have some merchants in camp. At the end of each map, there's a portal to a new one, with new random encounters scaled to your current level.
Maps could include outdoors, underdark areas, and dungeons — and maybe even some on other planes! Each map should basically be designed with one path through. (This is not about exploring or choices!)
Maybe there are "boss fights" every few (character) levels -- and perhaps a super-boss after level 12. But not essential.