Banks: Deposit Gathering and Loan Origination/Servicing (Banker/Loan Officer, etc.)
Investment Fund Management Firm: Operate Retail Mutual Funds or Private Hedge Funds (Portfolio Managers)
Personal Financial Advisors: Help Wealthy People Manage their investments for a percentage fee (~1%) of the Assets under Management (AUM)
Similar to the Law Firm and the Programmers it would be a cool add to be able to setup a firm with stocks, bonds, forex and commodity traders. The game has a good amount of retail. This would be an expansion to the services side.
I want to add to this as my actual job involves working in the Commercial Lending sector.
I think adding banks would be quite beneficial to gameplay, and it could actually be used as a means of controlling the wealth you gain on top of the current methods of Investment, Real Estate, and Negative Interest.
Banks in themselves would be pretty simple, you would need to attain depositors in order to lend out the money on deposit of course with interest accrued from payments made by those whom you lend to. I think that in order to make sure that this doesn’t runaway inflate your income, you could treat it similar to how you get payments in Real Estate: Large pool of funds returning a small amount per day (this is purely for gameplay balance reasons). Although the US has MANY rules in place on how Deposit institutions work, I think the biggest thing that would need to be implemented is Regulation O. In game you can be lent a maximum of $800,000 from Vantander and $40,000 from Jensen Capital. I think the same should apply, which is where Regulation O comes in. Insiders (aka you in this case as the owner of the bank) cannot be lent money in favorable ways against normal lending practices that customers receive, so when a bank lends to an insider it must be equally scrutinized just like any other person. With that all being said I think the best amount in this case would be $1,000,000 just because it is your bank after all.
I think there should also be Commercial Banks in game too. Now in real life Retail and Commercial banking typically go hand in hand, there are specialized Commercial banks out there though as well. I think this would be a cool opportunity in that you can be a depositor and lender to just the businesses in game.
I think for both business types there would need to be two jobs on top of Cleaning: Teller and Loan Officer. Tellers would require workstations that have Computers and a new appliance: Locked Cash Drawer. Loan Officers would only need workstations with Computers. I think we would also need new counter tops similar to the IRS Building in that not only would it have that glass separator, but also the space for a computer and Cash Drawer. Maybe this can be all built in one so you’re not getting 2 appliances? Just a thought.
Now the next thing I think would make this cool and unique is that in order to even start the Bank you’d need a Bank Charter and starting funds so that you may lend out. I would honestly say the best way to go is to make the starting cost a minimum of $50,000,000 or if you feel it needs to be higher then so be it but definitely not below that. You could treat it similar to the Investment Screen in that you’re putting your funds into it, but in this case there is no positive/negative rate of return, it is just placed in there and that’s it. If need be it can be removed.
In order for this to actually all work out, you’ll need to have people come in and bring money and generate loans. I think for the depositing side, the best thing would be to have Businesses/People start relationships with your Bank by having a daily allotment of their income “Deposited” to your institution. Normally all their money would be of course, but since this is a Business type in game I feel like things could get broken really fast so I feel like for a Person perhaps 5% of their daily income and a Business maybe 2%, of course adjusting for balance if need be. For Lending, I think it should be similar to how you oversee hiring, except in this case there’s a daily reminder to approve or deny extending credit to customers/businesses and a specific screen that lets you do so, and in order for it to not be overwhelming maybe at max 10 loans requesting approval can populate. The screen can show things like the person’s or Guarantor’s (for a business) Credit Score, which in the US ranges from 300 (Terrible) to 850 (Great, actually perfect lol), repayment terms, and maybe chance of Default and make that default a random (but low) chance. If a person or business Defaults it gets written off and you lose money, perhaps 60% of the total amount lent out.
Finally, how would all of this massive wall of text make you money? I think the best solution is to just make it so all the interest accrued turns into Profit (for you) and money for the bank to lend out at a 75%/25% ratio respectively.
I’m not sure who will even read this massive wall of text but I hope someone does, because Banking would be an amazing and unique business to own and run as it would be much more involved than just the set it and forget it that a lot of the businesses are now, plus it would add another thing to do in game where there sometimes isn’t much to do other than Sleep, Exercise, Watch TV, Play Video Games, rinse repeat.
Would love to know people’s thoughts!
A great idea for the middle, late stage of the game
Could also be interesting, if we could have the Stock purchase/sell/Issue 
Indeed - and there are a lot more votes on that topic too! 
Stock Market/ Angel Investing - Feature Suggestions - Big Ambitions Community (bigambitionsgame.com)