I found WordPress plugins to be a bit of a race to the bottom for many ideas (eg: something is already doing it for free) but I imagine there is a great market for enterprisey plugins.
There are also some excellent plugin markets for most eCommerce platforms if you've got the drive to develop in them. Companies using an eComm platform are likely to be actual businesses so they are willing to spend more money for a quality plugin.
Yes, the low hanging fruit has been plucked, sometimes a dozen times. But if you think of WordPress as a very simple method of self-hosting, there are lots of apps that have yet to be built.
The other option is building a SaaS app and using a single WordPress install or WordPress multisite as the platform. I thought this was a crazy idea a couple years ago. Now I know of dozens of apps that have WordPress as the back-end, and it works well.
There are also some excellent plugin markets for most eCommerce platforms if you've got the drive to develop in them. Companies using an eComm platform are likely to be actual businesses so they are willing to spend more money for a quality plugin.