Okay well, as per usual almost lol, I don't like this idea. Let me explain why:
Since there are plenty of things to do on your skyblock island that are pretty much risk-free, there would be no reason to pay the bankfee while doing those things, hence, players simply withdraw their money to save it. Then, whenever it's time to do something a little more dangerous (expanding the island or slaying mobs), players would simply store their money, do their thing, and withdraw it the moment they're done again. This is literally a way of saving money you shouldn't have been able to save, adding the fee is nice, yes, but it'd have to be crazy high to balance out the gains from only storing it at times you're at a risk of dying. And even if the fees were to balance this out. There wouldn't be a reason to use the bank in the first place because you'd end up with the same money either way (considering that you'd die, because if you were to survive everything, you end up with more). Hope I explained this well enough, I'll elaborate more if you want so :D
Then, you suggested a form of paying each other money through the bank, I'd much rather see this feature being implemented in the form of a command, if it were to be added in the first place. Yes it'd be nice I guess, but I'm honestly doing fine without it on my co-op island. Also, there isn't much reason to have a lot of money on skyblock anyways. I simply store up my items that are worth money for whenever I want to buy a spawner or whatever, and then sell everything in one batch, followed by me buying the spawner. Fully risk-free. If I'm able to come up with this, so will others.
Spinning of your idea, I was contemplating, what if rather than paying a fee to store money in the bank, you instead got paid to put money in the bank. This would work a bit like interest. The catch would be however that when you put your money in the bank, it remains inaccessible for a week or so. This means that if you don't need your money at that point in time, you can put it in the bank for a day, and get maybe 3% extra in a days time. This would stop people using it as a "Get out of jail free" card if they falling into the void and navigated to the hub.
This sounds somewhat fun, but in all honesty, a bit too complicated, especially when considering what it's actual purpose is. It's not like this'd be a major big feature or anything.