So, I was just recently in the Guildhall of the Derp Knights, and it looked no different from the Six Samurais one (Six Samurais was the guild I was in before). The Guildhalls look so bland, and are so small, and have no purpose except to buy trinkets. In order to make them cooler, and give them an actual purpose. For decorations, maybe allow the Guildmaster(s) to create or select banners? For example: Say I start my own guild, called the 7 Moons. The banner would be probably have 7 moons on it, and if I were to create a guild called the Blazing Knights, it's banner would probably be a Knight on fire. You could only create a banner from objects/shape supplied by OOO, so no profane banners can be made. Also, maybe add trophies? The way this would work is that when a member of the guild has done a certain thing, a trophy will be added to the guildhall in the empty room on the left. Or course, that room would need to be modified in order to have this make sense, or OOO could make an entirely new room in the guildhall for trophies.

As for the arena idea, maybe have something in the back of the guildhall that looks similar to the PvP thing in the arcade, exept green (the colour of guild chat). This costs no crowns or energy, and has no equipment restrictions. There is no prize, just like normal PvP (unless someone issues one).