Theory: Undoing the snap will only be a small part of Avengers: EndGame, not the whole movie.

And thoughts on what the "bigger threat" could be (in reference to the wording on the toy packaging).

The Russo Brothers are some of the KINGS of misdirection in the movie industry. They did it with infinity war and there is NO DOUBT they are doing it with Avengers: End Game.

Everyone is convinced the next movie will be all about bringing back the fallen heroes, but I would wager to bet that bringing them back will be half or less of the whole movie.

1. Action figures include tons of fallen heros

if everyone is brought back right at the end, why would kids want to play with the toys of those heros? If a character like Black Panther simply comes back to THANK and hug everyone-- would it really be worth making an action figure of him?

2. THE MOVIE IS THREE HOURS LONG

-Need I say more? We thought it'd take Thanos two movies to get the stones-- he did it in 1 and did the power stone off camera and the space stone within minutes. Three hours is plenty of time for each of the heros (at least the ones with action figures) to get their needed screen time, set up the bigger threat or/and set up tons of movies for the future.

3. There is a bigger threat.

This leak from one of the toys packaging, "Rocket teams up with a new crew to guard the galaxy from an even greater threat" confirms there is something more threatening in Avengers EndGame.

When I hear "a bigger threat" I am thinking a bigger threat than the snap, not Thanos himself-- though you could argue both- Thanos causing or becoming a threat bigger than the snap would only make sense. If everyone is gone, there is nothing threatening about that. It's a fact, a done deal. BUT if everyone is brought back AND you add heros that weren't before (Valkrie, Captain Marvel, Rescue, etc.) then Thanos would need to be stronger than before, a new villain would rise or the affects of the snap would have other cosmic implications that threaten the universe's very existence (I have another theory down below about that). However it happens, either with Thanos or something else, a "bigger threat" would render greater screen time.
4. Everyone was wrong the first time around.

-Everyone thought Tony Stark would die, they were all wrong. And in that same pattern-- Everyone thinks the movie will be all about about just bringing back fallen heros... I'd wager that the recovery will be a MUCH smaller part of this upcoming movie.

--SO... What could the movie spend more time on than just rescuing fallen heros?---

THANOS like we've never seen him before
-It was Loki who attacked NY, Not directly Thanos. And when Thanos did any of his attacks it was also focused on a goal, not just general evil. Thanos has always been calculated. Let's say he wasn't mortally wounded from the snap, the snap was undone (by The Avengers, an Eternal or whatever other means) and the great threat is still Thanos-- then he'd have nothing to lose and wage war with everything he possibly could have at his disposal in a rage to just kill everyone and everything. THAT would be a much more threatening Thanos than one who is just targeting small glowing rocks due to a mentality that made him (in his mind) make a difficult decision to change (or better) the universe. A Thanos like this will need screen time to build up.

Thanos would go on a rage because his plan was undone and becomes blood-thirsty to destroy absolutely everything. That would be amazing to see on screen and just as powerful of an evil as the snap was-- maybe worse because deaths done in hate are worse than in randomness.

Some other threat that is not Thanos at all
-This is unlikely, but hear me out-- The snap could have something else up, something that was set up right under our noses since Iron Man 1 and will be revealed in Captain Marvel. 10 years of misdirection seems kinda far fetched-- BUT if they did do it, it'd be the greatest bait and switch in cinema history.

In this theory he is just an ignorant part in the plan-- sure he is a big bad villain, but either way his snap was not the biggest problem, the snap just opened the door for that threat. If there was another threat, then that will be will either be some sort of space-time-continuum rip that threatens everything or an eternal or army outside our realm that was waiting for this realm to weaken to attack. Thanos' snap only affected OUR universe.. not the multiverse. This would be in the same kind of idea of how Oden's death brought Hela back-- how certain characters death or repositioning could bring other threats back, but on a grander scale. Red Scull is loose, are you sure Ultron is dead (the helmet Peter found in Spider-Man: Homecoming was glowing after all), Dormamu made a deal with Dr. Strange so with strange gone, heck, even he could come back-- the stones have a "mind of their own" and a "wisdom" so they could even be a threat-- the possibilities are endless-- and those enemies or concepts will take up half of the movie or more.

Unrestricted Thanos OR something else-- either way-- there is a greater threat than Thanos' snap and that is the threat the current heros need to get the fallen heros back for. They're not bringing them back just to have them back for funsies- they are bringing them back because they NEED them.

Something funny
-Okay, now this theory definitely is unlikely-- we all get pranked somehow and the movie has nothing to do with the snap. 3 hours of getting Rock Rolled would be both awful and hilarious. Or the original Avengers go back to the battle of New York to get Shwarma (spell?) and somehow forget they were on a mission and get stuck there. So that first Post credit scene is half the movie and the other half is them yelling at each other or saying how their stomach hurts (seriously, how does Thor eat human food?).