Content assist is one of the nice features of Eclipse text editors. But unfortunately, we can add only a single ContentAssistProcessor at a time to the ContentAssistant for a given content type. Still, JDT does a good job of showing multiple categories of content assist in Java editor.
I posted an enhancement to eclipse bugzilla, that will allow editors to support multiple categories of content assist processors. I hope it will make to eclipse sometime, meanwhile you can pick up the code from the bug entry.