First thought for the game is a better banking system.

My current thought is a server vault, with individual vaults for each character.

huh what? Could we get some context to this question?

preference. what do you think is best in an online game.

I also do not understand. Why are you talking about toons and banks in the same place? Is this bank for a toon? What is the distinction between a bank and a vault? Do banks have more than one vault?

In most MMOs, you have a bank, you can put things away and save them for later crafting supplies, alternate armor, what ever. A vault would be an actual storage area within the bank.
A server vault would be available to all characters on the account on that server, no matter their allegiance. An account vault would be accessible to all toons on an account no matter the server. A personal vault would only be accessible by that toon.

I like the option of One personal bank vault, a shared with your other toons on one server. This way an option to click on your user account may bring up your different characters each with an access to the characters inventory. If the server was to change I would think that the stored server would keep the character on the server with the inventory.

I think I'll ask a mod to move this to Gaming Discussion, it's not a GM Workshop question.