On the subject of the iron bank:
Perhaps there could be a cap on the interest rate of the iron bank? Perhaps between 250,000-500,000 thousand coins or something around there? It would still yield a decent interest rate, but it would be capped at a certain number. Depositing coins over the cap could still be an option for storing money if you desired but the return would peak.
What if your relations with Braavos, weather if it's the actual town, faction or leader, determined how good or bad your interest rate was at the bank? If experience has taught me anything, bankers really start to kiss your behind the more you have invested with them so I have no idea how this would be implemented. Maybe the more money you have with them, the better the interest rate? That could make things interesting. Just a thought.