I certainly hope I never stop feeling like there's a lot more to learn. Once you start feeling that way, the challenges and the pleasure in overcoming them will most certainly end, which doesn't necessarily have anything to do with how much there's really left to learn.
There are so many things that are not solved yet. AI, in a human context comes to mind. Speach recognition, text to speach, image recognition are solved problems. Combining them into a seemingless intelligent experience (robot is optional), is not solved.
Master that, and you might become rich if you do.
We have the hardware so you could build a self balancing robot, that walks around and picks up trash by the road. It would be incredibly hard to build it yourself, but it's possible. However, we don't have the HW that would let you build a robot that can read a random book and the assignment and then write original essays which would consistently get's a 95 - 100% in English 101 at Harvard.