If Warhammer 40K is infact in a Bottle within the University of Altdorf then End Times(which destroyed the Old World with Chaos ripping into Altdorf during it’s destruction) may be Chaos’s greatest victory as it now has direct access to the Universe that spawned Slaanesh and the Eye of Terror!
Considering Slaanesh’s Greater Daemons are the Keepers of Secrets it stands to reason that one of them grabbed the 40K Universe during End Times and is as of right now joining a bunch of Fiends of Slaanesh in chasing the Tyranids towards the Galaxy that spawned their master with their faces(which should be gigantic since they are phasing their faces through the bottle rather than opening a portal that shrinks them to get to one of the Planets).
Imagine the Tyranids minding their own business only for a couple giant faces(some resembling Anteaters) the size of a galaxy popping up and slurping them up! More than enough reason to invade the 40K Universe!
The Slaaneshi Keepers of Secrets need to live up to their name or else Tzeentch is going to smash the bottle(for the sake of Change) and kill Slaanesh in the process before the Slaaneshi even freed him from the Elves!
Of course the 40K Universe being in a bottle lends to questions on “What are the C’tan?” Then again they apparently came in reaction to the hatred the Necrons had towards the Old Ones who decided to pay the bottle connected to the Warp a visit so that they could create Eldar.
The C’tan claim to have once faced the Old Ones and were sealed away by them so they definitely have a grudge against them.
Since the Old Ones following the War in Heaven disappeared I would assume their original Universe was besieged by the C’tan and the Old Ones left upon which they discovered the Warhammer Fantasy Universe and seeded it with life only to be driven off by the Chaos Gods forcing them to hide in a Magic Bottle only for their old enemies to be drawn to those who despise them!
Naturally the Old Ones out of desperation brought the Orks in to bail them out only for the C’tan to continue winning wiping them out.
The C’tan known as the Deceiver claimed that the C’tan were losing before the C’tan hid themselves but the Deceiver is the Deceiver after all. In all likelihood the C’tan were winning and it was the Old Ones who hid themselves before the C’tan tracked the hate of those the Old Ones had spurned and manifested themselves within Altdorf University’s Bottle!
With all that in mind it’s clear that the C’tan didn’t seal away the Warp because they despised it but because they didn’t want their prey to escape again! Once they were extinct their next order of business would naturally be to get Abaddon(who clearly has a C’tan Shard as a Topknot and was led to his Daemon Sword by the Deceiver himself) to remove all of the Necrons machines intended to seal off the Warp so that they can invade it next!
Once the C’tan learn of the Worlds of Age of Sigmar they will invade them as well! The Deceiver(in his Laughing God disguise)'s servants from his Realm of Aelindrach the Mandrakes will be the Deceiver’s agents in this function.
The Realm of Death is coming for the Realm of Chaos!