You want to make money with a SaaS business. But you need an idea. Well, you don't.
You want to make a lot of money, so you'd better prepare for it. For instance, be ready to accept payments, and store and deliver invoices. But you want to really make a lot of money, right ? So you develop a scalable application to deal with that load of payments and invoices. And since a lot of other businesses have the same needs, you offer your application with a subscription model.
You now have a SaaS and you can continue to offer services that you need yourself. Accepting payments and invoicing was just the beginning. You need backups, so you offer a backup service. You need monitoring, and a status page. You need to send email, you need to deploy updates, you need to host your source code, ... you have a lot of needs, so you can offer a lot.
To start, you identify one of those needs, and you offer a solution, to yourself and also to others.