I'd say centre or wing. Reason - players of his calibre need to have their rep careers considered in their contract negs, and therefore position is crucial. In Inglis' case he would not represent at either fullback of 5/8. He would at centre or wing, or as a wide running back rower.
He needs quality service too. At Melbourne, Cronk and Slater gave him that. In rep sides, mainly Thurston. At Souths, it was mainly Taylor, sometimes Sandow. Taylor game him good ball but not much running space. Inglis needs space, in traffic he's just a big truck. He needs space to use his fend, footwork and then speed. He has good ball skills, but is not a classical centre that will set up his winger aka Matt Gidley.
Inglis will not slim down again, even though he still has speed. One on one, he's a monster, and I'd love to see him on the wing outside someone like Cooper or Tahu. It will never happen.
His future will be back row.