Clearly the fighting around the Eye of Terror wouldn't just stop. It would be an ongoing war.
So yes it was a "minor Chaos victory" for game purposes but the story would go on.
And since it's said to happen as "the 41st Millenium draws to a close" future events would happen in M42.
Armageddon may be in a quiet phase right now but eventually the war would heat up again.
The Tau will continue to expand, especially since the Imperium's resources are needed elsewhere.
Then there's always whatever the Orks, Tyranids, Dark Eldar and Necrons are up to.
* The Imperial Fleet would focus a majority of it's forces currently in the area within the Cadian system.
They would bombard the Chaos held areas and clear territory for addtional troops to land.
Space Marine chapters would be most effective in this because it would require less in
troop numbers and each would have a higher life expectantcy after making planetfall, especially in a hot LZ.
As many Grey Knights as possible would be brought in to do the anti-Chaos special forces job they do.
The Imperial forces would regain all of Cadia, but at great cost.
* Adepta Sororitas orders in the area would be sent in to secure the cities and other valued areas
after the Grey Knights took out the fierce daemonic prescence.
* Combined with above the Agripinaa Sector and Belis Corona Sub-Sector would continue to act as
a rear area for Imperial fleets. Rotating in new one while withdrawing others for repairs and rearmament.
* The three areas would be placed in a sort of lockdown as the higher ups attempted to
bring the situation under control. However long that would take.
* The Ork's "Green Kroosade" would be having fun with MorDakka Prime and Orks on that world would
be primarily concerned with building new stuff to git da humies wit. They would decide they didn't need
the deal with Abaddon anymore as they'd have da good bits all to themselves.
* Ulthwe Craftworld having lost Eldrad would fight ever harder to stop Chaos from spreading more than it had.
The Eldar in general would be working to make sure their gains on Belial IV. Now that Altansar has been found
they'll probably stay in the area since they seem to bee Daemon fighting experts. This could result in a
complete return to Eldar control. So the question is can they somehow release it from the EOT
and reclaim one of their old worlds?
* Typhus would consolidate his control over Ulthor and turn it into a Daemonworld of his own design.
Perhaps even becoming a Deamon Prince himself. Unless being the Herald of Nugle suits him and his God better.
* Agripinaa is going to have food supply problems since it can't depend on Yaynor, Ulthor, and Dentor anymore.
Supplies will have to be brought in. Causing further problems Chaos forces will know this and every supply shipment
that makes it in will be paid for in blood.
* Tau Expantion will continue and even more worlds will be brought in making them stronger and able to better defend
worlds allready taken. Imperial policy may be just to keep the Grox dung at shoe level and not much more than that.