Captain America: Civil War showed us that Tony hasn’t let go of his issues with his father yet (and he’ll never get the chance to, thanks to Bucky), and there are some clear parallels between how Tony feels about Howard Stark and how Harley feels about the father who walked out on him.
Neither of them has received a clear resolution to their issues yet, so maybe Avengers 4 will finally give them what they have been looking for since meeting one another.
Of course, after everything that has happened in the Marvel Cinematic Universe over the course of the last decade, we already know that Tony Stark and Harley Keener have far more in common than Tony would let on during their first encounter.