I've been considering this idea for quite awhile, and decided to unleash it.

Guild Masters will have the ability to customize the Guild Training Halls with whatever they want. This could range from a mission-like attempt or a simple room full of infinitely respawning monsters to practice. This could give a much different feel than simple enemies such as Shufflebots, as well as provide endless user-made content for entertainment.

Construction of Rooms
The rooms to be created are not simple like 'Purchase 3F East Wing'. The idea is for it to be full customizable, deciding where you wish to implant your walls, floors, ceilings, and the base spot of where monsters shall spawn (or placing a monster spawner.) Say you wanted a practice of zombies and bombies. You could place Grim Totems inside of your 'mission' or training hall room. Then, these undead would infinitely respawn. (Or, an easier way, a monster spawner.)
The placement of objects during construction is very important to 'make it your own'. Users could go hands-on to create a neat little mission with many different things. These objects are not to be placed via 'pick it up as knight, move it, go to control panel and publish'. A free camera to fly around wherever your soul desires it to be is exactly the idea I'm thinking. (faster, smoother, and easier process.) These items should also be normal-mission items like switches, buttons, gates, party buttons, spikes, poisonous thorn bushes, candles, statues, totems, etc. (And yes, with party buttons I mean multiple people of your guild could go with you.)

Missions
This is exactly where those of whom are on Treasure Vault should be. You should be able to create multiple guild missions, just like the normal mission board. This goes for those missions that are all speech, and could be whatever you want. Saving knights? Ok. Journal of one whom has entered and hasn't returned? Ok. That's the point. A whole storyline could be behind these, or something a simple as 'Training of Undead Type'. The missions where you enter materials/weapons should be available. These materials shall be deposited to the 'Guild Spark' Token Trader. There should be rewards like 'Guild Sparks' and 'Guild Sparks of Life'. More preferably, these rewards should be named after the guild, or whatever name the guild master chooses.

Rewards & Shop
This is where I'll go in depth about the emphasized text earlier. The material missions previously mentioned should have the materials obtained placed in the Guild Spark Store. Say somebody had some materials to put in the mission, got some guild sparks and guild sparks of life, then exchanged those Guild Sparks for valuable WEAPONS, ARMOR, & MATERIALS they want. Do you want the Obsidian Carbine in the store? Maybe an easy go at it would be exchanging your materials to get all the guild sparks necessary! Or, if you're feeling like fighting, you could earn those guild sparks yourself by running missions! This would encourage guild members to use the guild training hall missions, and exchange their materials for the ones other guild members deposited themselves. Another idea for ways to get Guild Sparks is to directly sell your items for Guild Sparks! (This would deposit what you sold to the Guild Spark Store, and get you some Sparks based on the # star item it was.) This would become a whole currency that only had use within the guild, and would be treasured like crowns and normal tokens. Into Guild Sparks of Life. I shall abbreviate it with GSoF most of the time for convenience. GSoF's can be earned from missions, purchased with crowns (or energy, to go to the energy well). All the money used to purchase these will be sent directly to the guild treasury, giving that guild member another chance at going and whipping those monsters. Also, that guild member would get a warm feeling in his intestines (or stomach) that he got ressurected AND donated to the guild treasury. Think of how easily guilds could earn crowns this way! These GSoF's should be only usable during guild missions, and not regular missions, and reside in your arsenal until you're ready to use them. Also, being able to trade them to other guild members (assuming they only had use in one guild), you would be able to sell them for cheaper than normal price (just like AH).

Event Exclusives
Cool! This would easily raise the rate of which prize boxes were bought from Supply Depot. Referring back to Contruction of Rooms, these would be normally event exclusive monsters to be placed in your guild missions! Would you love to fight infinite Gourdlings from a Gourdling Spawner? Who knows, perhaps you'd rather battle that Black Kat? Maybe you're just in the mood to mash some Creep Cakes and Batterbots! In my opinion, I'd rather fight that Vile Dreadnaught and Choking Bombie, but whatever you want! It's up to you, whatever you'd like to put in that mission! Prize boxes should have a 1% or lower chance of containing a box-related monster or monster spawner. This would really pick up the fun of running through these missions by fighting otherwise exotic monsters that can only be encountered on 7-14 day events! Guild members would enjoy this, and guild masters would have a desire to purchase a large number of the prize boxes to see if they can get any of these rare creatures!

- More ideas for this will be brought up later if I think of, or remember, anymore.