Picking up shortly after the events of Thor: Ragnarok, the third Avengers story introduces Thanos and his children (the "Black Order" from Marvel Comics) after they've already boarded and taken over a starship carrying Asgardian survivors. We're told, not shown, twice that Thanos only killed "half" of the remaining Asgardians of the Marvel Cinematic Universe which doesn't quite fit with what is shown (nothing but dead Asgardians and the entire ship blowing up via the power of an Infinity Stone).
Presumably, and this will need to be explained in May 2019's untitled Avengers 4, Thor: Ragnarok stars Valkyrie (Tessa Thompson) and Korg (Taika Waititi) were offered the chance to leave with half the ship's population before this. Why Thanos needs to kill half of them just after the events of Ragnarok is up for debate, but this part of the story isn't the only important bit of lore skipped here.
Before this assault, Thanos had already collected one of the six Infinity Stones. Again, audiences are not shown this happening but instead it's revealed in dialogue later in the movie between the indestructible Thor and the Guardians of the Galaxy where Odinson tells the cosmic team of misfits that Xandar was "decimated" a week earlier by Thanos where he acquired the Power Stone from the Nova Corps - where audiences last saw it at the end of the first Guardians movie.
This raises some questions. How did Thanos so easily take Xandar without the aid of a Power Stone and why didn't the Guardians help out since they're all about answering distress signals (and how didn't they know this happened a week earlier)? Well, the first part is easy to answer and doubles down on a problem of the first Guardians of the Galaxy film. In that story, Ronan the Accuser's (Lee Pace) goal is to attack and destroy Xandar and its Nova Corps space police force because his people, the Kree, have been long at war with them. Except, this isn't really shown. Xandar has virtually no military presence in GOTG 1. They have no effective warships, planetary defences, or anything that can stop a giant brick from falling to the sky (Ronan literally flies a giant, non-weaponized twisty-looking brick and drops it on the surface).
Okay, so that answers itself. Xandar is just a planet that doesn't make sense. How could it have been at war with the Kree Empire without any war machines? Don't tell me those little web-making ships (called "Star Blasters") are what they fought the Kree with in an intergalactic war. And don't tell me in the years since Guardians of the Galaxy highlighted their ineffectiveness that they still didn't beef up planet security. In the end though, it doesn't matter because Avengers: Infinity War throws all of that away. We had a movie dedicated to saving Xandar and with a line of dialogue all of that accomplishment is wiped (the same goes for the theme of Thor: Ragnarok where Asgard is its people and not a place, half being wiped out off screen), presumably by the same forces that couldn't even defeat the less advanced heroes of Earth (until Thanos arrived of course).
Similar to the case of the missing Asgardians, we've long suspected that the unseen events of Avengers: Infinity War (specifically, the collection of the purple Power Stone from Nova Corps HQ) would serve for future storytelling. Here it's the impetus to the creation of the real Nova Corps as they are depicted in Marvel Comics - not as pilots of web-making starfighters, but as super-powered, flight-capable cosmic heroes. Perhaps the complete uselessness of Xandar and the old Nova Corps gives rise to the Xandarian Worldmind from the source material, an artificial entity that powers the Nova Corps. This would setup the inevitable Nova movie which I've personally been championing for the better part of a decade, allowing for an Earthly hero to join the cause (Richard Rider or Sam Alexander, or both).
In that Nova movie, which Marvel Studios president of production Kevin Feige says is likely coming sooner rather than later, the siege of Xandar could be depicted on screen in all its glory and maybe this time they'll put up a fight.
And before that, Captain Marvel will introduce younger versions of Ronan the Accuser and his ally Korath (Djimon Hounsou) in March 2019 which we expect could retcon their characters to add more personality and complexity to the villains. Ronan, for instance, becomes a bit of a hero in the comics and it'd be great to see him more than a crazed, brick-flying maniac who's easily distracted by dance moves.
Much like the deaths of half of the Avengers heroes in Infinity War, get ready for a wave of retcons coming with Avengers 4 and the other upcoming films which could play with different parts of the MCU timeline and offer more context and perspectives to some of the things we've already seen. Xandar could be re-envisioned to something more like the comics, Asgard could be entirely rebuilt, and villains could become heroes while new heroes are introduced and old heroes are killed off.