Fuck now I need to decide if I should try and slog through the most relevant (not shit) HH books I haven't read or just skip to the end.
If there's any big world shattering revelations (which hasn't to put it mildly been a strong point of the HH series, see the Emperors crazy ex-wife that tried to run away with their children) then it won't be hard to find out since every 40k youtuber that does anything even remotely lore related will be doing a video on it.
So if you're curious you can easily find out and then start the from the start and work your way through. But if you're the person that wants to find out for yourself then definitely read the last book first.
I'd have to imagine that GW would find some way to continue writing HH stories, including the aftermath if need be.
I thought the aftermath was a foregone conclusion. They might even go all the way and show what happened to the Primarchs.
Otherwise there isn't really many other big events unless they want to do another War of the Beast and just pull major previously unmentioned and nearly lore shattering events out of nowhere.
They could do the Reign of Blood, that could be interesting but that seems like it would be more political intrigue (at first) then big chunky space marines shooting each other.
There's several Imperial crusades that could be fleshed out, although most of those were Imperial Guard heavy (given how rare Space Marines are) so that might not be a good replacement either.
Or you know they can finally get around to expanding the current 40k lore which really really needs some love. I mean we had like a handful of books all almost exclusively focused on Guilliman to cover a 200 hundred year crusade (which I don't believe they even really did) that pretty much solved all of the Imperiums issues after the cicatrix maledictum.
Talk about wasting a good opportunity. 7th edition ends with the galaxy literally ripped in 2, half the Imperium's territory was lost, Chaos raids everywhere and then the first novel they release is set 200 years later everything is fine now (better even) because Primaris and then Gilly goes off to slap fight Morty. I get why they did that because they wanted a lore justification for why every single Space Marine chapter now has Primaris (i.e. why you should totally buy Primaris Blood Angels models despite the Blood Angels being stuck on the other side of the great rift completely cut off and isolated from the Imperium) but in terms of story and world building it really was meh.