At the end of Avengers: Infinity War, Iron Man was left stuck on Titan with Nebula (and a lot of dust). Whether the two of them are trapped there or can use the Guardians of the Galaxy ship to escape is unclear, especially as that could very well have been destroyed when Thanos pulled down a Moon on Earth's Mightiest Heroes. However, one fan may have now unearthed a big spoiler for Avengers 4.
As you can see below, a behind the scenes interview which was seemingly shot when Avengers 4 was filming shows a blonde-haired Robert Downey Jr. on a set which is quite clearly Shuri's lab in Wakanda. This no doubt means that we can expect to see the two scientists come together in order to try and figure something out - perhaps it's here that they discover how to travel back in time?
Right now, your guess is as good as ours but with a trailer for Avengers 4 expected to be released in November, here's hoping things start becoming clearer soon, especially as the movie is still such a mystery.