People should lend directly to each other, or have sensibly priced investments, or people should have simple bank accounts that don't surprise you with charges, or be able to change FX cheaply. Those kinds of ideas are pretty easy to suggest for outsiders, and the answer is always "I'm gonna make a website that's better than the incumbent".
To be fair, startup FS sites have tended to be easier to navigate, and focusing on an area like FX does create a simpler user experience.
The question is whether any money can be made. For instance we now have a load of challenger banks that have a slick app for sending money to people. But how much is the customer actually worth? Do they stick around or open a whole bunch due to it being so easy? Also what are you gonna upsell them on other than the metal card?
What about the lending markets? How come Lending Club has issues making money as the premier business in the direct lending category?
What about the investment businesss? Competing on price is generally something they tell you not to do in business school, but that seems to be the main value proposition as far as I can tell. I reckon Google could jump in here though. They have credibility among the general populace as being pretty smart with ML, and I bet they could morph their bank into a super hedge fund.
The FX niche, I don't know how TransferWise are doing, but it's fairly easily encroached on by the challenger banks. The backend is fully commoditized, it's a question if getting customers. Which is probably why TW are offering bank account like services.