Let’s be honest, stash space will always be an issue, especially with even more things to store in the next expansion. I think it would be useful for each character to have a personal stash tab that’s not shared with the rest of the account, just to store their own gear options. If and when we get build loadouts this would be even more valuable to keep each character’s alternate gear apart from the others.
It isn’t a totally new concept either, Diablo II Resurrected has a personal bank per character and the shared bank.