I'm sure it's already been suggested, but it would be nice if we could deposit electrum into the vault, as well as withdraw smaller amounts.
Currently, you have to withdraw your entire fortune even if you only want enough to buy a ration, and can't deposit anything back into the vault.
As a result, my character routinely walks around with more money in their pockets, than in their Scrooge McDuck vault.
A more controversial suggestion (please hear me out):
I know it would be hated by some people, so like with 'Buttslut Mode', 'Silly Mode' and 'Short Pregnancy Mode' - I would like an option for 'Child Support Mode'.
In this mode, after you become a Marcher-Baron, Carmen/Gwyn would visit you to politely explain that you'll have to pay for any children you have in the nursery. The limited nursery funds must be reserved for the poor people of Hawkethorne, not wealthy land barons. You're welcome to use the nursery, but will have to pay from now on, etc.
From a roleplay/story perspective, I think this would make more sense, particularly when you have dozens of children in the nursery.
I also think it would give motivation to players in the late-game who have already bought everything there is to buy, to keep earning money. Instead of money becoming meaningless, it becomes necessary if you want to have hundreds of children.
I would suggest that every 10 days, to coincide with Pay Day, money is automatically removed from your vault/person to pay for your children. Carmen will kindly cover your expenses if you can't afford a payment once in a while, but if you miss 2 payments in a row, you receive a Bad End where you're ran out of town.
Since it's a round number, let's say each child costs 100 electrum every 10 days. If you have 100 kids, that's 10,000 every 10 days, which is more than manageable by that stage in the game.
You're welcome to adapt/change any of the details, of course. That's just a rough outline of what I had in mind. I'm not possessive of the idea at all, but I'd be happy to write any of the required text, if it would help it to be implemented.