Being able to section off the bank and have different permission levels for each section would be a great start. Of course, it does not matter how much space is there, unless bottomless which is not happening, since many guilds have the issue of players dumping junk in there which requires leadership to manage the inventory one way or another.
Of course, Zos could implement a system where guilds could purchase additional space. 1 mill for an additional 100 spaces and have it double for each additional 100 spaces up to a max of 2k total guild bank space. It would be very reasonable at that price.