I honestly enjoy OPs alot, And sometimes only log on just for an OP battle

OPs for the win.
Spires for the loss.
OPs could just be so awsome with some work! *dreams* Take the Spire idea...Break it down, And use that system for towers and add it to OP defense 'instead' of guards, Have the towers maybe be able to do a bit of damage...
But make it so the owning guilds (or allies of the guild) Crafters/Diggers have to build the towers... and in order to take the OP the towers must be destroyed in the 2 hour period(Limit the amount of towers ofc so you can't have a super un defeatable OP

You can even make it so crafters/diggers can do/repair damage durring the fights... (This too would have to be limited, Atleast on the defending side(they would need to be OP tagged to do it))... Crafters could make ranged weapons and Ammo to install on the tower... And thats what would do the damage(Imagine,A launcher on it... that would need to be reloaded! that would get crafters into the battles more!)... Or an Armor maker could make Armor to increase its defense!
Get the diggers/crafters invovled some...
Those are just a few ideas... I could think of so much more for OPs

[edit] You could even have jewel crafters make resist jewels that would make magic do less damag to the tower.[/edit]