Cities: Skylines - Financial Districts Review (dragonadamant)
A modest recommendation if you have real money and Cities money to burn. I know this isn't a full expansion, but even Airports looks feature-rich in comparison. I do think Financial Districts' income is similarly impressive, but it's not as stable as Airports was for me.
I tried the new office buildings once, forgotten about until long after I'd finished the achievements. I want to call the buildings I've seen ugly, and they certainly look nondescript and lacking in variety, but they're easy enough to ignore while still making use of the banking features. You can insider-trade stocks pertaining to your own industries, so if you wanted to finally simulate Horse Economy 3, now's your chance, and if you understand the principle of Buy Low, Sell Hay, you'll probably make a good profit from your stocks, though I'd make a backup save in case of severe losses. This is worth an evening's entertainment for easy achievements I got in half an hour.
While my investments' income seemed to dip too wildly between extreme positive and extreme negative values to be consistently useful, I didn't find the investment category's leveling quotas difficult to reach, and gains from super-successful sales do count toward stock-exchange level-ups without just stopping at the beginning of the next level's quota bar. (The investment menu is easy to reach from the same menu you go to for loans and budgeting if you don't want to hunt your entire map to remember where you put your stock exchange, which itself is located on the first page of the Unique Buildings menu, not with the other banks.)
Oh, and as for the "Cash Flow" achievement if you want to do it the easy way, don't bother spawning a tsunami, if you can even do so on your map. Just build a small bank on the coast at sea level, then construct a huge dirt mountain right off the coast. The displaced water will flood your bank, it doesn't even have to be a sustained flood, and the achievement should instantly appear.