Although this is not mediatorship per se, the following passage refers to an (eventual) universal reconciliation between parties as implied in mediatorship:
"He is the head of the body, the church; he is the beginning and the firstborn from among the dead, so that in all things he might have the supremacy. For God was pleased to have all his fullness dwell in him, and through him to reconcile to himself all things, whether things on earth or things in heaven, by making peace through his blood, shed on the cross. Once you were alienated from God and were enemies in your minds because of your evil behavior. But now he has reconciled you by Christ's physical body through death to present you holy in his sight, without blemish and free from accusation" (Colossians 1:18-22).