Since we seem to be fully dependent on our midfielders for progressing the ball into the final third(If this is by choice or due to our fullbacks being less than impressive on the ball, I don't know), we must get a CDM/CM that can pass better than the likes of Ndidi or Rice. If we go for a more static version like Koopmeiners or someone more mobile, like Zakaria would depend on what we do with the rest of the team. If we get a new LB that offers more going forward, or a mobile CB etc would impact what we actually need.
Due to PP somewhat erratic style, I would go for someone used to playing in a structured and defensive system. Someone like Partey, which coincidentally is the best (subjective) player potentially available, and the one with the most balanced set of attributes. If Tonali was available we should go for him, but not likely.