I'd like to see the guild halls as more than just a place you return to every 5 levels for quests. For battle classes, I think adding a training hall where you can hone your skills would be the perfect solution.
I imagine such training would be several stringent classes, where a certain technique would be taught. Rather than guildhests, where they advise you to follow mechanics but can easily be bruteforced, these would be designed so that deviating from the plan leads to failure. These would also be solo instances with NPCs, as this is more about your own skills than those of your friends.
So, that all said, what sort of classes would you like to see taught in such an area? I have a few for WHM.
Lesson One: Heavy
This lesson would involve using your Stone spell to strategically inflict Heavy on opponents. At the start, a ranged enemy will be kiting your NPC companion, attacking him and staying out of range. Inflicting Heavy allows your partner to close in and attack. Later in the fight, the enemy prepares to use a powerful short ranged move (Think the slime from Wanderer's Palace), inflicting Heavy allows your NPC to safely kite it and avoid the attack.
Lesson Two: Cure
A fairly simple excercise, this would teach that the best offense can be a good defense. A powerful enemy that takes little damage from the player's attacks, and an NPC that can damage it but has no self-healing. Keep him alive to win.
Lesson Three: Cleric's Stance
This lesson teaches you stance dancing, requiring you to defeat some enemies while keeping allies alive. The effects of the stance are exaggerated for this lesson, so your spells have no effect if used in the wrong stance.
Lesson Four: Fluid Aura
This lesson has you bound, with a tank making use of flash a short distance away, and monsters that immediately aggro you upon spawning. Simply use Fluid Aura to knock them back to the tank. Later on, you must make your way to a switch with an enemy attacking you, make use of Fluid Aura to give yourself time to use the switch.
And so on and so forth.