You could build anything with Goaddon. Feel free to read through our project ideas. Some could be usefull for your ecommerce company, others could be offered to ecommerce companies by you as a service provider.
By subscribing to the Goexplore addon you get a website that online shops can use to sign up.
By subscribing to the Gofetch addon you enable the merchants to import their orders and products into your database.
If you install the explorer-at-goexplore repository, your website even get an orders page that you can extend with informations about customers subscription changes.
So by now you have the customers in your database. On top of that you need functionality that enables the customers to sign in and manage their subscriptions. How to authenticate them is a challange for you to solve.
Additionally you need to somehow integrate with the same payment providers as the ecommerce platforms does, so you can capture payments.
This kind of project could be approached from many different angles, all of them quite complex.