In addition to the Hall of the Novice and Mentor system, I think they should have added new Guildhests for some of the recurring mechanics in late-game content. The following are not covered by the current Guildhests:
- Tethers
- Stacking to split damage
- Divebombs
- Enemies that ignore enmity
- Tank Swaps
- DPS checks
- Hiding behind objects to avoid severe damage