Some people actually blog about ideas they'd like to see: http://blog.stevepoland.com/

One way might be to combine a platform (PC/Mobile/Voice) with a communication medium (RSS/Twitter/Video) and think about ways people could use a new combination. For example, Voice + RSS = people 'reading' the web from listening to a phone.

Another way might be if you are using a service already but there is a serious 'feature gap' that you could address, i.e. how to find out what Twitter users are linking to? Sure you would have to de-code tinyurl addresses, what else would you have to do to accurately report that figure? (Idea was suggested by Merlin Mann; he is another guy who frequently posts ideas he'd like to see).