Well I mean, the fact that Spider-Man, Black Panther, and Star-Lord are confirmed to be getting sequels means they at least need to return.
One theory I saw going around said those who died in the snap are likely having their souls held within the Soul Stone, kinda like how the Space Stone transported Red Skull back in the 40s. Strange definitely knew something.
Personally I'm banking on Avengers 4 being the last hoorah for the original crew before they metaphorically pass the torch. Whether they retire or die fighting Thanos remains to be seen, but I'm banking on at least one of them sacrificing themselves, either Tony or Cap probably. The former to go full circle with being the MCU's first hero, and the latter to reflect him sacrificing himself again like in the end of Captain America: The First Avenger.
Those who died pre-snap are doomed though, imo. With Vision dead though they can do an MCU variation of Disassembled and House of M though.