I like the idea. Though you would basically have to switch to each job and re-queue, to prove you have a high enough iLevel for each. I don't think the game could just take your word on that. Only after you have been "checked" on each job, then you can queue as multiple roles.

Regarding that idea of changing roles if someone in your party drops, I would like that only if there is a vote and the other players agree. I don't want to see a DPS drop, and then have the Tank decide "Oh goodie, I'll be DPS now!"