'And when all things have been subjected to him, then shall the Son also himself be subjected to him who did subject all things to him, that God may be all in all.'
And then once the Son, as glorified man, having already received all authority and power (Matthew 28:19), has all finally subjected to Him, then He Himself will subject Himself as man's Redeemer to the Godhead. And God will be all in all.
We must note carefully here the terminology. Christ has been raised and received Kingly Rule, then His own are raised with Him (1 Corinthians 15:23), then having abolished all authorities and powers (1 Corinthians 15:24), as the Son (1 Corinthians 15:28), He delivers the Kingly Rule to God the Father (1 Corinthians 15:24). Then God is all in all. In this is made clear the mission of the Son, to come as the Anointed of God to bring about the redemption of all things, so that He might deliver all to the Father, at which point the Godhead become all in all.
'God will be all in all.' Once Christ's second coming has brought about the resurrection, and all enemies have been defeated, including death, and He has then handed all things over to God the Father, God will be all in all. God will be everything in all creation. He will be the sum of all things, to all. He will be the all sustaining sufficiency in all. There will be no need for a Mediator. God will be all, in all.
Does this then make the Son inferior to the Father? That is a human question. There is no inferiority in the Godhead. Jesus shares with the Father the glory that He had with Him before the world was (John 17:5), when with the Spirit they had been all in all. But the Son was commissioned by the Godhead to the task of redemption, and for that purpose emptied Himself (Philippians 2:6-7), making Himself lower than the angels (Hebrews 2:8-9), making Himself man, that dying on the cross of humiliation and shame, He might restore to God the Father the people whom He had chosen. And in His glorified manhood, having paid the price of sin, He was raised, and He was given again the name above every name, the name of Yahweh, so that all was subjected to Him, that in the end His re-establishment as 'Lord' (Yahweh) might be to the glory of God the Father (Philippians 2:9-11). But in the end it is God (not just God the Father) Who is all in all.