Creating "bank toons" and constantly having to log in and out to retrieve and store items is not only tedious, it's more work for the login servers and the game database. Therefore, I propose the following...
Allow players to exchange up to 5 character slots for 100 inventory and 100 bank spaces each. Tokens that unlock these spaces can then be mailed to that account to be used by any character on that account.
For example, if I only wanted to play one character, I could trade in 5 character slots for 5 tokens representing a total of 500 inventory and 500 bank spaces. I could then apply these tokens to my one character. Should I want even more space, the inventory and bank upgrade paths for that character would still be available, as well as any storage a horse would provide.
In the end, it will make the players happier, and will keep the ESO DB smaller and servers will deal with less login/logout traffic.
Just my two cents