Have you tried SEM Rush yet? It is a good place to build a list of several thousand keywords that you do not currently get traffic for by giving you competitive analysis on sites that compete with you.
There is also Hit Tail which builds lists of long tail terms that people already use to search for your site, so that you can add these terms to the list of things you want to optimize for.
Let me know what you think about these two tools, so that I can get an idea of what to recommend next if these do not fit the bill
