I like it, but the system can be easily abused. Here is two examples that I have witnessed in another mmorpg.
1. Three players from the same guild join a dungeon as 1 healer and 2 dps, the tank is a random player. They reach the final boss and just before they engage the final boss, the tank got kicked. Why did the tank get kicked? The tank got kicked, because one of the dps player wanted to make 100% sure that he would get the tank weapon/armour if it were to drop. So the dps player changes into a tank, they get another random dps, they kill the boss and a tank item drops. You are probably asking yourself: "Why didnt the dps tank from the start?" Because like alot of players in this game, they have tank alts, but they do not want to tank to earn the gear.
2. Instant queue, every dps or healer would queue as tank, join the dungeon and change their class into a dps or healer.
I think the only way this would work is, if players join the DF as a full group (like via a Party Finder), otherwise this feature will be abused and the forums will be filled with QQ of players complaining about how someone kicked them get (insert item) or made them wait, because they queued as tank and played as another role.
This does not mean, I dislike this suggestion, I like it. But the ways players can abuse this system can damage the community and the game itself.
