There are a lot of small companies offering that - a mix of SaaS and custom development.
In a previous company, we built a SaaS platform (based on Odoo, a kind of semi-open-source web version of SAP) which allowed clients to purchase changes to any part of the application. Unlike in a typical SaaS, each client had its isolated copy of the app/database/etc (though all running on our cloud), so they could change anything. Some changed nothing, others have whole new components and business flows.
In a previous company, we built a SaaS platform (based on Odoo, a kind of semi-open-source web version of SAP) which allowed clients to purchase changes to any part of the application. Unlike in a typical SaaS, each client had its isolated copy of the app/database/etc (though all running on our cloud), so they could change anything. Some changed nothing, others have whole new components and business flows.