You are on the right track there. I have been thinking of something like this for a long time. You are essentially building a SaaS framework as a service. I do not know how easy this would be to build, but I do believe this could have a real future - especially when you make it super easy for a small company to create a SaaS app.

It is interesting that the first module you plan to build is the marketing site. I may have started with something more core to the SaaS experience. But then again, you might be correct in doing so.

You also do not need to build all the parts of the ecosystem yourself. You could integrate with apps that provide very specific SaaS services like baremetrics for saas metrics, getdrip for lifecycle emails etc