There is some precedence. We have ****** difficulty level when doing leves. When doing a dungeon we could have the same options.

(*) Easy mode: This is what we have by default (60 minute run).
(**) Normal mode: This is what a lot of us try to do (speed run).
(***) Hard Mode: Timer set to 30 minutes, and difficulty of all mobs increased significantly.
(****) Very Hard: Timer set to 25 minutes, and difficulty of all mobs greatly increased.
(*****) Suicidal: Timer set to 15 minutes, and difficulty of all mobs massively increased.

Every one can do easy mode. Normal mode takes a bit of practice but is doable.
Hard mode would entail a level of co-operation and team-play, as well as highly developed skills, in order to succeed.
Very hard mode would be border-line impossible. Feasible if everything goes perfectly.
Suicidal is impossible, until the level cap is increased (at level 75 it would be equivalent to hard).

Rewards would scale up obviously with difficulty. At suicidal mode, the drops would be much more common.