A lot of your customers ask for the same things: a MySQL database. An Apache server. A Mail server. Many others. Right now if we want that we buy a VM of adequate size and we install what we need, possibly through one of your prepackaged images. This works okay, but you can improve on that drastically, saving us money and time, as well as saving yourselves hardware costs.
You are now running 1000s of MySQL instances, 1000s of Apache servers. Sometimes this is needed for security reasons, but usually those instances run trivial and repeating tasks for different customers, like running Wordpress sites. This is inefficient: we are wasting your resources doing the same things over and over. Why not sell access, say, a MySQL database owned and managed by you for our droplets? Or access to a mail server, complete with some groupware suite installed? You are already doing that for DNS and backups, why not for other things?
This way I won't need to supersize my VM to host a database and save some $$$ by using services you host for me, and you won't need quite as much hardware to keep your services going. It's a win-win, and really a no-brainer.