It seems like this is a viable way to get 250 extra inventory slots by using guild banks. I have thought about this before, but I wanted to see if anyone has done anything like this. And I also want to know if there's a flaw to this that (game mechanic wise, not people's behavior wise) I'm unaware of. (I've never started a guild, so I'm uncertain if there's a mechanism that I don't know about.)
The idea is to split guild bank inventory between 10 people across 5 guilds. You have to have the 10 people to create a guild and gain bank access. The 10 people should be folks you trust who will stay active, because everyone will have bank privileges. You have to create 5 guilds with the same members. (Is that possible?)
Set permissions for the guild banks so that only 2 people have access to each guild bank. Give the 2 people the same guild privileges for the guild they have bank access to. Your inventory is mixed with one other person's, but that shouldn't be too much of a hassle for a close knit group of 10 folks.
It does limit you from joining other guilds, but you could fix that by having less inventory. (Have the same structure, but pair with more people across fewer guilds.)
So has anyone tried anything like this?