I see two possible plot points taking place pretty early on during the movie or even in the after credits scenes of one of the films due to release before End-Game.
Tony Stark and Nebula are seen in the Benatar, the guardians most recent ship.. obviously escaping titan and making their way back to some form of a sanctuary or back to earth.
Since the Sovereign are still seeking revenge against the Guardians since they stole the Anulax batteries that they were tasked to defend from the inter-dimensional tentacle monster... then later in the movie wiping out thousands of their drone ships, it is not unlikely that the ship will be intercepted or captured by the sovereign which will later introduce Adam Warlock who was teased in the Guardians 2 after credits scene.
The other possibility is Captain Marvel, depending on the result of her film she may be in that region of space whilst the infinity war was taking place, probably fighting the Skrull... We know that Nick Fury sent her a message at the end of infinity war, so it's not unlikely that she finds tony stark and helps him return to earth.
Or maybe thor finds him? seing as he can control the bi-frost with Stormbreaker now.