I think this is the dilemma most teams have. I reckon that we could all go through every team in the league and point out a couple of positions that aren't good enough for either starters or depth.
ATM I think we're missing a full-back or wing-back - possibly more complex, depending on how frank sets up. I expect he'll suit what we have already in his approach, so either one or both LWB/RWB - ideally one to rotate/push Porro/Udogie, and one to backup (leaving some allowance for if Spence doesn't step up)