van Dijk has a ceiling that can be fulfilled with the right players and manager around him. He's already physical and a strong attacking presence up front with pace. His biggest weakness to me is gettin caught out of position during a counter, which is to be fair not much of his doing because being tall he's expected to be up top, and the DM and another defender covering the defence.
If we get Mendy, we have a physically formidable defence and harder to mark in corners and set pieces. Then again, Mangala was meant to be a physical presence.