I think people are talking about immigration, though it's a thorny issue, partly because Korea has been virtually monoethnic for a very long time. But I think Korea will eventually have to open up more: the current population crisis is so dire that I don't think Korea is in a position to choose its medicine.

(I don't think immigration alone will solve the problem, BTW - Korea should obviously fix its horrendous work culture, education, and runaway housing prices.)