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?
It's really ill-advised to solicit your vision from anyone. In my 20 years of building, investing and supporting tech companies, I don't know of a single success story that has it's origins in someone with your approach.
Running a tech startup is incredibly hard. It demands sacrifices few are truly able to make and come with it tremendous risks that most people are unwilling to take.
It sounds to me as if you want the startup life because you have an impression of what it's about but haven't yet experienced it first-hand.
I'd encourage you to first join an early-stage startup. Developers are incredibly in-demand. Find an entrepreneur who has some experience, funding and a compelling vision that you believe in and get to know what the journey is really like.