This appears to be an open problem: see question 42258 at MO. However, Wraith [1979, _Generic Galois theory of local rings_ ] speculated that the fppf local rings are ‘algebraically closed local rings in an appropriate sense’:
> Since a ring of polynomials in many variables is finitely presented and faithfully flat over its subring of symmetric polynomials, one may deduce that the inverse image of the generic commutative ring in the fppf topos has the property that monic polynomials over it split into linear factors.