The more I think about it, the more I'm convinced: Variety is bad. Variety is evil. Variety is the natural enemy of balanced core gameplay.
To be honest, this is why I like rogue-like games. Absolutely no (or very little) story, yet the game is perfect because of the gameplay. Every play through is basically the same, but with different experiences due to randomization of rooms, enemies, and pickups. I envision that a "rogue mode" would do great in a Descent game since the gameplay is already awesome and the classic SP is already repetitive. A rogue mode I envision would have multiple randomly generated mines that you would explore just like you would do in D1/D2 then destroy by blowing up a reactor. Each new mine would have it's own color palette. you only get one life and your score is recorded upon death. You would only get a quick save file that deletes when you make another save. When you restart again, you start from scratch. I imagine that this would foster it's own leaderboards as players compete to get high scores...
...all this from a repetitive cycle of awesome gameplay. sure I imagine it would be fun to add a non-competitive "for fun" version in which you can visit certain seeds, tweak some parameters of the game, or even play co-op (for fun or to get a high score).
There's no need, IMO, to add a bunch of bells and whistles to make up for the absence of good gameplay. Just fix the gameplay. Make it stand out in a way that isn't gimmicky. Sure, RPG unlocks and things like that are cool, but once you unlock it all, you'll quickly lose interest (at least I do). Having a rogue mode with no unlockable weapons/perks/powerups/whatever forces players to realize this moodd is purely about skill