I have no idea how it will happen, but I strongly doubt they will wait until after Thanos is defeated to bring the heroes back. The whole theme of Infinity War was that the heroes lost because they weren't together. That seems like a pretty obvious setup for a scene where all the heroes are fighting together.
The third movie avoided a lot of things the fans wanted but won't get until the fourth movie: Tony and Steve weren't onscreen together, Banner didn't turn back into the Hulk, most of the people Thanos harmed didn't get any payback, we didn't see Vision functioning without the stone in his head. All that is likely to happen next year.
Seeing Tony and Steve together again at last will be a big focus of the movie, obviously, but if the dusted heroes play no part in defeating Thanos then it will imply that the original Avengers are incredibly essential and most of the newer heroes aren't essential at all, and I doubt that's the impression Feige wants us to have.