I think what you're looking for is just higher/ better rewards. The adventurer in need bonus is specifically for the class most needed to fill out groups among the players already queued. It's not arbitrarily tank all the time. It stays as tank because there are far fewer people filling that role. It's designed to incentive people to play tanks (or whatever role is in need), though clearly it doesn't actually work well enough to create any sort of balance as it remains tilted toward tanks. Maybe it is having some effect, just not a significant one.