I think the easiest way is to implement it and then build upon it. I know nothing about coding, but it would seem like the architecture for the guild bank could possibly be utilized to make an account bank. I would start with the basic unbound items like potions, talis, mats, dyes, BOE gear ect. This would free up initial space. But what about people that like to hold on to event items and sets of armor or cosmetic items that are bound? I don't think there would be an impact on game play if this was allowed, even if items were changed to bind to account. The only questionable thing in my mind would be allowing RvR currency to be account items and therefore interchanged between characters on an account. And if this would be allowed, would you dare allow them to be used cross realm? I would currently vote no on the account currency.
How are the Account Bank slots earned or unlocked? Through just gold purchases, achievements, quest-lines, different for each vault page/slot? Please lets not use RvR currency to buy them. Maybe X number of slots open up based on some type of a achievement, make us earn them instead of instantly giving them away and make it fun. So many slots earned by completing each dungeon (one time reward)? Or so many slots earned for each armor set completed, i.e. Redeye set 5 slots(one-time reward)? World Event? Gold Bag Item, token be turned in to the banker to unlock X bank slots (only so many slots can be opened)? X number of slots over with each character that hits XX rr level? Tome unlocks?
Throw out some ideas guys, or even troll mine
-CannonFodder
