Legendary quests are already out there. There are a whole lot of epic versions of old heroic quests that I never see ran : Sentinels chain, for instance.
For legendary quests, use the "epic template" of the quest and lower the CR of mobs appropriately.
Give people who complete a legendary TR a cosmetic hat according to the class with which it was completed and the same benefits as a regular TR.
Is it commercially sound? Why not!
Make legendary quests free for VIP or sell them in bundles for non VIP. This should encourage subscription, which IMHO, is the clean non sketchy way of making business (non of this otto boxes, aka cheat boxes, or destiny keys or XP tomes or XP pots...seriously turbine! Take off the pickpocket costume!).
Heroics are more fun than epics (for me, before some captain obvious pops up), there are more possible builds. Imagine beating epic quests on heroic characters. I call that the real epic!
PS - If you wish make a side progression for the plot for legendary TRs...how about the underdark in heroic toons? :O