I'm familiar with lean methodology and aware that ideas don't really mean much, but at the same time I'd like to work on some big problem that's compelling. I know that I'll probably pivot a lot but just to start out what general problem would you reccomend I try and solve?
You have development expertise. Consider immersing yourself in a variety of development environments (I mean corporate, not the technical definition) perhaps by contracting your services out. In many areas, contractors are in huge demand. What does this get you? -- Immersion. Over the course of a few months or a year, you'll start to recognize certain types of problems or challenges that others are faced with -- provided you are looking for them! Which cause the most pain? Which are you most passionate about? Are these intractable or do you have a solution? It's out of this maelstrom of complexity that great ideas can arise. With your new armamentarium of experience, you can now explain the problem, attest to its commonality or perhaps even ubiquity and offer a solution. Now, it's fine to take your hypothesis and to test it further against others in the field, but you need that hypothesis in the first place. Unlike Einstein's Theories of Relativity, your ideas will likely not come out of thin air, which is precisely why you need to gain that experience. (A good complementary alternative, as Tom notes, is to join a startup, which will give you a bird's-eye view into how startups work.)