Simply add more incentives to play as tank or healer and that will get queues moving. Mounts are nice, but what about other features.

Here are a few suggestions for rewards when players choose to tank or heal:

1. RNG-based rare item drops exclusively for tanks and healers after completing a dungeon (ie. Rare weapons and gear).
2. Increased tomes, gil, seals for duties. (Current adventurer-in-need rewards are too low).
3. MGP card drops after completion of duties.
4. Minion prizes exclusively for tanks and healers for completing X number of duties.
5. Random housing item drop for completing trials/mentor roulette.

These are just some ideas, but it's definitely a start!