Almost every AAA MMO has guild banks even Neverwinter by cryptic and pwe.
You have guild banks in LOTR via the chest storage in guild hall home etc.
I do realize that to code it, you would need to do more then code it due to space and stuff.
However you can make the guild bank with bank tabs that you unlock for free via guild levels as well as money sink.
Lastly turbine to pay for the guild banks dev time You can charge guilds if they want the 3,4,5,6,7 bank tabs unlocked then via the DDO store they have to buy the unlocks for the guild this is win/win for Turbine to make money with and as extra guild perk at certain levels etc.