I have thought about this myself.

I am from Africa and needless to say, we don't have an abundance of VC funds for startups.

I have used products live Revolut and Monzo before and loved them, there is an opportunity being missed in places like Africa and other developing economies that have a huge growing youthful middle class that hates the way the established banks operate and is comfortable with technology.

I spent most of my free time last year building a backend that can handle some core banking functions, I already did research on the licensing and knew I would never be able to raise that money just with an idea and business plan so to keep myself going and also have something to show potential VC's I decided to start small, things like airtime top-up, bill payments etc

Break it down , look at the bits that you can build out and get some initial customers while you look to raise money, you don't have to go all out and build a whole bank, start small, licensing for things like e-wallets and money transfer is not unreasonably expensive, you can then go on to build on top of that

Good luck.