Wait, wait, wait, everyone slow down. We're discussing what's going to change, and I don't even know what the fuck is going on! Is there some tangible product behind this teaser trailer of sorts? Is it an update to the lore/retcons galore or a new novel? I am very confused. Is GWS end-times-ing us?
Warhammer 40k is moving into it's 8th edition soon, either June or July I forget which. The timeline is progressing past 999 M41 and into M42, which has normally been the hardcap on event times.
The 13th Black Crusade has begun properly, and will presumably be a big part of the new setting. Cadia has been destroyed, cracked in pieces by Abaddon. With Cadia gone the Galaxy has been riven in twain by warp storms, and half the Imperium has been cut off from Terra except through small stable warp routes. The newly awoken Guilliman and Archmagos Cawl have devised a way to more rapidly produce marines that Cawl has been working on for 8000+ years, as well as new gear for these new marines to use, and now a bunch will be getting made and deployed to hold back Chaos. The Ynnead aligned Eldar are nominally allied with the Imperium against Chaos. What's going on with everyone else we still don't know.
Mechanically the game is being simplified. Special rules will be found in the units that have them's rules, vehicles and MCs will now work the same way more or less, and a bunch of smaller rules are gone or folded into the main rules they form exceptions to. In total the new core rules are supposed to be just 17 pages. To hit rolls are now done on a fixed number, cover improves a units saving throw, armour pen is gone and replaced by save reducers ala Fantasy/AoS/Old 40k. Pistols can be fired in close quarters. Blah-de-blah-de-blah, bunch of changes, core concept stays the same.
Everything is supposed to be getting new rules on the release day, and it's supposed to be updated yearly, rather than the erratic schedule currently used.
I am surprised you aren't aware of this, 8th edition has been anticipated since peopel realised how shitty 7th was balance wise, and people have been wanting Abaddon to get off his arse and do stuff for decades.