Prior to the release of the trailer, it's being claimed The Avengers: Annihilation is the official title for the sequel to Infinity War, and also that a plot description has leaked online.
I've been tipped off that a user on Reddit from France posted information offering that he has a friend who works in the movie business where the friend has been provided The Avengers 4 title along with an official description of the plot.
According to the Reddit user, the person who provided the info works as a distributor in France and says he/she has been given official information stating The Avengers: Annihilation is the title along with the plot description.
Regarding The Avengers 4 plot synopsis, the info for Avengers: Annihilation describes it as:
"when facing with an even bigger threat than the Infinity War, the outnumbered Avengers have to find the most unlikely ally of all."
The Redditor noted that he is from France and that the description might not be the best translation, but that is what he has been told.
Worth a mention is that the Redditor posted it on the /r/marvelstudiosspoilers Reddit, but after being harassed and cursed at by other Reddit users, he deleted it. The Reddit user has a relatively high karma as well and claims not to be making up the info.
Now regarding The Avengers: Annihilation as the official title, it has been rumored to be the title in the past.
Regarding the plot synopsis, it does actually fit in with previous descriptions of the movie.
POTENTIAL AVENGERS 4 SPOILERS FOLLOW
Merchandise packaging has listed both Thor and Rocket Raccoon as taking on new villains, as has a tie-in Infinity War comic book (note: bold my own follows).
Regarding the Thor toy description, which features a look at the new costumes, it offers: "The Asgardian Thor wields the mighty Stormbreaker in his battle against new foes."
Likewise, the description for Rocket Raccoon says: "Rocket teams up with a new crew to guard the galaxy from an even greater threat."
For the Infinity War tie-in novel, author Brandon T. Snider recently tweeted the cover and back cover of his "MARVEL's Avengers: Infinity War: The Cosmic Quest Volume Two: Aftermath" novel, which happens to mention a new foe. Interestingly enough, the description also reveals that Selvig and Darcy Lewis have survived the Snappening:
Doctor Erik Selvig and his associate Darcy Lewis are determined to find and understand the cause of the devastation. With a team of friends both old and new, they embark on a pursuit of knowledge, eager to discover the links between their own pasts and the stones that decimated half of humanity: the Infinity Stones. Little do they know that a new foe who threatens to destroy them all is looming….
TODAY’S THE DAY! The search for the truth begins in @Marvel’s #AvengersInfinityWar: Cosmic Quest Volume #2: AFTERMATH. Who are the Science Avengers?! Pick up a copy and find out. https://t.co/hpI06Z4jFG pic.twitter.com/eYStCiWuzZ— Brandon T. Snider (@BrandonTSnider) November 27, 2018
So we have at least three, and now possible four, descriptions that offer The Avengers 4 will feature a villain, perhaps even greater than Thanos in Infinity War.
The Avengers: Annihilation as the title does fit with that possibility if you are familiar with the Marvel Cosmic comic book event from 2005/6, but obviously it remains to be seen if any of the comic will be a part of the movie or obviously if any of this information is true.
Regarding The Avengers: Annihilation plot synopsis from above which mentions the "most unlikely ally of all," I'll let you speculate.
The Avengers 4 trailer is said to be dropping soon, so hopefully, we'll get some sort of inclination as to where the story is going.
The Avengers 4
The film has a May 3, 2019 release directed by Joe and Anthony Russo and stars Robert Downey Jr., Josh Brolin, Chris Evans, Chris Hemsworth, Scarlett Johansson, Mark Ruffalo, Jeremy Renner, Karen Gillan, Bradley Cooper, Chris Pratt, Zoe Saldana, Dave Bautista, Tom Holland, Benedict Cumberbatch, Paul Rudd, Evangeline Lilly, Sebastian Stan, Chadwick Boseman, Don Cheadle, Danai Gurira, Letitia Wright and Brie Larson.
The movie is a sequel to Infinity War where the Great Titan left the universe in shambles by killing off half the population including the Avengers. Now Marvel's super heroes must suit up perhaps one last time to make things right by taking down Thanos once and for all.