I love Sif. She's dust at the moment, but I'm sure that won't matter after Avengers 4.
Why do u think Loki is coming back? What would they do with him? Hes run his course redemption arc and all. Maybe he makes an appearance in avengers 4 but it wont be a ressurected Loki. It will be a past or alternate and after the movie Loki his still dead. Theres nothing really left to do with him
The most likely way to reverse the dusting of half the universe is to use the Time Stone to turn back time to before Thanos did it. If they turn it back just a little further they can also bring back characters like Loki, Heimdall, Gamora and Vision who died from causes other than the dusting.
Some people just need to die. Loki and Heimdall have no further purpose to serve unless someone thinks thor is gonna be the first character to get a 4th film. Gamora and the dusted heroes are inside the soul gem(Didnt Russos confirm this or maybe just for gamora) They probaly just have to be free'd. Vision can be rebuilt and Loki and Heimdall should stay dead.
Well, I'm assuming a little too much from the comic story there. In that case Nebula used the time stone to revert everything back to 24 hours earlier - essentially to just before the story started. I'm assuming something similar here.
Bringing him back doesn't mean he has to continue to be used. On the other hand, it would be bad business to not use him until the audience proves they are tired of him.
My guess is that Loki and Heimdall stay dead, Gamora gets brought back, and Vision gets re-created but with no memories of his past life. (If he could be simply rebuilt and rebooted, Wanda wouldn't have been so devastated by his death.)
All of the dusted people are coming back, obviously, and I think a time reversal is most likely since all of the bodies are destroyed. I'm not sure if a full year will pass "in story" between the two movies or if 4 will follow right on after 3, but any events between now and then would get removed from continuity (including the Ant-Man movie).
A lot of bad things could be undone, but so could a lot of good things. A big question would be whether people would remember the things that happened during the time that gets rewound. Based on what's happened on Agents of SHIELD I'm guessing they would.
If they bring everyone back by reversing time, I expect there will be a price to be paid in the form of some major accomplishment or good thing getting erased from reality by the rewind.