As you all know, the guild hall is just a place to hang out with your guild in private...and pretty bland, but the layout is quite nice and even more so for decorations and add-ons!
First, I propose a mini-auction house in the guild hall, a little place that allows us access to the auction house from the guild hall. This will be the most useful add-on, in my opinion, and will make guild mates want to hang out at the guild hall more often.
My second proposal is to have themed decoration packs you could purchase with energy, such as Jungle-themed, Fire-themed, and Ice-themed. Adding this to mini-auction house idea, I'm sure many people will purchase themed decoration packs for the guild hall, and your guild mates will love it.
Will this render Haven useless? No, because people would still have to gather in Haven to meet new knights for recruitment or friending as well as trading.
The guild hall would be used much more often and would be a cozy hang out with more eye pleasing things to look out and more to actually do while in it. Any other ideas would be appreciated.
I would like to propose yourself posting in the massive thread that encompasses all of your ideas found here ----> http://forums.spiralknights.com/en/node/40011