I'm beginning to wonder if Mercury should be added to the list of potential destinations for HSF.
Of course, there are disadvantages: Mercury is closest to the Sun and there would be radiation hazards. I wouldn't be surprised if there were an issue of delta V.
And Mercury doesn't have an atmosphere, so a lander could just use rockets. Whereas at Mars NASA is grappling with slowing something big through the atmosphere.
A sample-return from both worlds might confirm or deny this theory by enabling the analysis the isotopes in the two planets' crusts to see if they might have come from the same part of the proto-solar nebula.