Massive online collaboration for solving open problem in theoretical computer science
Polymath projects seems to succeed when a breakthrough happens, and one is trying to optimize the result of the breakthrough or come up with simpler or better proof. See https://en.wikipedia.org/wiki/Polymath_Project#Problems_solved. As such, you would have to pick a problem of this nature in CS. The only one that comes immediately to mind is improving the constant in matrix multiplication https://en.wikipedia.org/wiki/Matrix_multiplication#Algorithms_for_efficient_matrix_multiplication, which is currently at 2.4... But frankly, I am not sure enough people care about it enough to work on it...

Questions for which I would expect polymath to fail miserably: P=NP, online optimality, UGC, etc.