Just to clarify, BS is a canonical example, but all I need is a PDE linear in second spatial derivative. So to meet your requirement I need a linear parabolic PDE a la heat equation (ie the unknown and all its spatial derivatives are in first power + the first derivative of time). Then the second spatial derivative “smooths out” all local spikes and dips - as time goes by.
I believe local volatility models qualify too, as long as they are linear in second derivative .
