Why hasn’t anyone thought of having a bank? Available at T3 we could transfer funds to a specifically modified Vault, or a Vault with only gold in it, and obtain interest. The Traders already have price rises and falls, a bank can raise interest rates or lower them based on how much spending a player does.
I do agree that someday the vault should upgrade; a bank would be a cool T3 or T4 option. I think at T2 or T3 also it probably should be the building where city guards could be hired and the murdering drunks (ugh!) could be jailed as part of a justice system to deal with that ongoing pain point.
Maybe it starts out as a vault, upgrades to a jail, upgrades again to a bank that conveys trader/market/pub/tax benefits and offers more gold storage. Though vault > jail > bank feels like a weird progression it’s actually not historically too far off. Maybe it could have two upgrade options depending which one you still need, I dunno. Would be a fun thing.
I also think the bank upgrade could shorten building times and farm development town-wide, because IRL that’s actually the main “job” banks do.
Like the forager huts, upgrade and then click on what you want it to be? It then stays that way for the rest of the game?