This is a great list and nicely expands "(b) overlooked problems." In his book "Innovation and Entrepreneurship" Peter Drucker offered the following observation on the frame of mind need to spot opportunities for innovation.
"Innovation requires us to systematically identify changes that have already occurred but whose full effects have not yet been felt, and then to look at them as opportunities. It also requires existing companies to abandon rather than defend yesterday."
He goes on to suggest seven places to search systematically for opportunities
Weak Link In Existing Process
Industry Or Market Structure Change
Demographics: Size, Age Structure
New Zeitgeist: Perception, Mood, Meaning