I would like to see a way to bank in a gold bar which you could purchase for 250 million. It can be sold without loss. However, the risk is when attacked, either an opponent's steal or attack had a small chance (5% or so) to steal/win a bar like spoils of war. I think this would be interesting and help promote pvp outside of a set war. And if you don't like it, you can still bank in bronze bars and suffer a loss when you sell.
It would be easier to just store your money in allies. What would be the point in the PvP risk when there is no PvP in the first place?