Honestly, I thought about doing it.
It could be a nice passive business to be a small payment gateway provider. It's not rocket science, I could implement it alone.
You invest some money to make it compliant with the security standards of the sector, charge less than all the other fat companies and you have another income stream.
The only cons is you have to abide to so much stupid regulation in order for it to be legal (not secure, I'm fine with the private rules to ensure safety in running the service), and then making sure your customers are not doing something your government doesn't want them to do (like cleaning "dirty" money) and identify your customers (KYC).
I looked also into starting a bank (you can do it with just 1mln in some places!) but you run into similar issues.
It's a legal nightmare; if there was no government, I'd definitely do it.