The best motivation for topics should come from what people are searching for on your site. This will allow those that may have flutter through and have not find something the first time to make a return.

Usually dealing dealing with your focus, yes Twitter can be a great source to go through the trending topics: WordPress, #wordpress and #wp.

Sometimes bouncing ideas from other bloggers whom may not have covered a topic to your satisfaction also works. You can give them a link back while mentioning how you came to think of such a topic. This is a win-win situation as you may get traffic back from them.