I'm not a beta tester, so I wonder how this DLC is going to work. If I have for instance 2B of cash, in what bank is it by default or is it under the mattress like it has been in the game before?

Like all bankers know, managing liquidity is one of the most critical things in managing a bank, and I think that in the banking DLC the player should be able to manage the liquidity and credit risk, which means there should be a central bank and other cash facilities (this is where my 2B should be initially invested by the bank), combined with liquid bonds (paying a bit better interest rate than cash (liquidity + default risk) and non-liquid bonds (again motivated by liquidity risk and maybe higher default risk) and naturally loans, and naturally there should also be an option to buy stocks with some limitations based on the banks own capital (for instance 2x banks own capital/shareholders equity).
One thing I hope you'll never do (and that hasn't been even proposed here) is introducing options, that would ruin the game, as it is too easy to control the stock markets in this game and the payoffs would be astronomical of doing so..