They tried a Guild bank many years ago on test. All we know is it did work as intended according to the Devs, I don't recall any issues reported by the player base on test. So the question now is was it's failure due to limitations of Flash, does this mean one of the reasons they went the unity route was because it would allow a Guild bank to be implemented.
Personally I do not see how a Guild bank would help solve the storing items in star. Firstly collectively some guilds must have 100's on millions if not billions of resources in star. Secondly it does not solve it for the independent players, Thirdly you lose all your resources if you get kicked from the guild.