When I worked on a custom CMS the killer feature (according to clients) was the ability to upload and resize images (which they can then insert to their text) i

When I worked on a custom CMS the killer feature (according to clients) was the ability to upload and resize images (which they can then insert to their text) in just a few seconds. If you make a service, with an API and wrappers/scripts for common CMSs I think you can get some money out of that.