My theory is the “bait and switch” one.
The Avengers that weren’t dusted - Captain America, Iron Man, Black Widow and so on - are the old timers. And the actors’ contracts with Marvel are all due to expire with the next film. The ones that were dusted are all the newer ones who have films even definitely in the pipeline still (Spiderman, GOTG crew) or whose contracts still have films left on them (Dr. Strange, Bucky).
So the dusted characters have to return, the undusted ones are expendable. I think by the end of Avengers 4 we might see some kind of exchange having happened, although I haven’t thought through how that would work
But my money is on some kind of Dr. Strange trickery - his pre-dusting words (“it had to be this way”) suggests that when he gave the stone to Thanos it was all part of a larger plan.
The “real” deaths - I reckon Loki and Heimdall are gone for good but Gamora will be back (again, she has more films on her contract...) I think there will be some Soul Stone trickery involved there that allows her to be resurrected.