From An Introduction to the Theory of Surreal Numbers:
[...] Thus the reader has the opportunity which is all too rare nowadays of getting to the surface and tackling interesting original problems without having to learn a huge amount of material in advance.
What other math fields would be in the same class as this one? What other math fields wouldn't require learning a huge amount of material in advance and would allow one to tackle interesting original problems?
Ramsey theory on the integers has research problems in every chapter (presumably still open) and I think it's pretty accessible to most levels.