I've been putting a lot of thought into what Turoq will most likely do next and have come to some conclusions.
First, an aside, I think that with his greater capacity for long term planning and choosing what is best for him in the long run over what causes the most short term advantage his domain will be less of a draw to powerful subordinates, and he will have to put in more effort than normal into fostering the growth of his own subordinates in such a way as to be sure that he won't have to kill them for plotting or being a threat to him should he display weakness.
As such he will have fewer native subordinate heroes than most Chaos does, and will try to make up this deficit with mercenaries and people on loan for the tech he offers.
We could try to turn his hired troops against him, we have berserk gas and telepaths.
Though he is likely aware of the unreliability of hired or allied Chaos, and as such will plan for them to be restricted and killed, either placing them in the riskiest positions in battle, or using them in an entirely separate force.
To turn them against him we don't need to deceive him, we only need to cause a diplomatic or communications breakdown between him and his allies. It might be as simple as telling them that he plans to sacrifice them for victory and asking them to think about their interactions with Turoq and the role he gave them in the war, because preparations against being betrayed look an awful lot like preparations to betray.
Thinking on what Turoq's actual plan of attack is, I have some ideas.
In the category of plans he doesn't want to use:
I think Turoq's final backup plan is discorporating after death and corrupting us or Dragon's Nest from the inside. He has a critical advantage in doing so given his impressive insight into the human mind, the lack of forcing self destructive tendencies in people he induces to worship him. I don't think he'd continue to try to rule from his Daemon world unless the destruction of his physical body is coupled with a major military victory, enough that the aftermath of this battle isn't leaving him open to other rivals.
if he survives but loses his domain and military he's gotten ahold of our tech and able to run and use that as a bargaining chip to work his way into the power structure of another Chaos polity after his defeat.
At which point he will inevitably become an even larger threat than he is now.
When we think of plans he actually wants to use the question then becomes where he will strike The Imperial Trust and how he intends to parlay that into an ideal victory for him.
He needs to take out Midgard or Vanaheim to win a lasting military victory and Avernus to win a favor victory. He has been watching our battles thus far, and saw the war on two fronts, he might have even seen the destruction of Garkill, or at least been able to access the Imperial Trust's records of the event.
We also need to remember that we've been discussing diversionary attacks and denying the flank extensively and the attacker has the initiative with those tactics, just because he can see through other's diversions doesn't mean he can't employ them himself.
I see a few smart targets for him to take, He could try an all out assault on Vanaheim, he has enough Psyker superiority to affect fleet battles, and if he has a gimmick or trick he thinks will work then he could easily succeed. For example if his crew corrupting raid rituals were more effective than the inquisition caught then he could threaten the Trust into concentrating them at Vanaheim, then induce them to betrayal all at once.
And I have vaguely suspected that Vanaheim is harbouring a coven of Rogue psykers since we learned that they were more psykic than normal. If Turoq has positive confirmation of my vague suspicion he could try some sacrificial ritual to corrupt them and get them to target the shipyards.
I don't think he'll commit to a heavy attack on Vanaheim, he's been hurt badly in Naval conflicts with us before, and will be reluctant to engage in another, but he might send a diversionary fleet of transports filled with sacrifices and ritualists to compromise Vanahiem's ability to build the maximum number of ships.
In short, If we see a fleet heading to Vanaheim we should send most or all of the psykers who are good at disrupting rituals.
But those ritual resources would be equally well used in another attack, so I don't think we'll see this.
He could target Midgard, I'm essentially sure that he can get past their orbital defences as it is, at which point his best option is to decide to dig in and turn it into a slugging match for however long it takes for the Orks to blow their tops, and we get to see what corruption and Sorcery can do to a planetary population in a multi year siege. Alternatively he could get an exterminatus weapon rather than fighting it out on the ground, either by using Sorcery for the task, stealing one from us, or buying one from one of the Dark Mechanicus forge worlds in the area.
If he heads for Midgard we're going to want to divine for the presence and location of an exterminatus weapon in his fleet and possibly put a higher priority on fleet response than the other locations he might go to.
We should also see if it's possible to be absolutely sure he cannot capture any of ours.
We should also prepare to be stuck drawing out the battle again to try to stop him from using his exterminatus weaponry, but if so will be unable to do so in the unnatural stilted and half committed way that we did against the Abomination invasion.
He seems to be at least somewhat focused on combating our technological advantage, so he might put a high priority on attacking those worlds that produce the most technologically necessary Material types. I don't think he'll go after Muspelheim, but he might go after Svartalfheim or Niflheim. In which case I have no idea which he'd go after, but I'm not sure how serious of a threat it would be in any case, Svartalfheim has impressive defences, while Niflheim is likely much better defended than it appears.
The next most likely target he might have is trying to either draw out fleets away from the core worlds, or induce rebellion out of feelings of abandonment in a subsector through preliminary attacks right before the final battle. I don't think he'll do this, if he were going to his raids would have mostly been concentrated on a single subsector.
Finally, we need to consider that it might not be sustainable for him to not attack Avernus. He runs a theocracy, and is buying support from other theocracies, presumably ones worshipping the same god, unless he has a specific purpose for the abilities of one of the other cults, and his god has offered an unspecified but large amount of favor to any who manage to wipe us off the planet. Not just the mercenaries he is hiring, but also his own subordinates, will challenge any plan of battle that doesn't move them personally closer to sharing in that reward. So aside from his need to guard against betrayal by his mercenaries, he will also need to satisfy them. Or appear to be satisfying them.
He can do this by forming them into a detached force to create a diversionary attack on the Helheim system, they are in a separate force so he doesn't need to worry about betrayal(even more important if the Mercs have ships, you don't want a naval betrayal when it looks like you're losing the ground war), They die, leaving him with fewer long term problems and the ability to deliver on removing troublesome subordinates for other polities, he gets a distraction of the Avernite forces to allow him to inflict immense casualties like what happened in the War On Two Fronts.
In conclusion I think Turoq's target is Midgard, with a diversionary attack on Avernus or the moons of Cumea, which one depends on whether his support troops are more motivated by gaining tech or favor.
He will most likely try to time things to either prevent us from supporting Dragon's nest against the Orkz, or to take advantage of their attacks to prevent us from following up on his defeat.
We need to think about what kinds of personal levers he could get against the commanders of his support units, and how we could subvert them. Notably I think we should have Ridcully present for the battle, and able to look for ways to turn Turoq's leverage over his sub-commanders against him.
@Durin
1.What are Turoq's motivations for the attack on us, what specifically does he prioritize gaining in a successful attack against us? dunno maybe don't answer this, people are saying it's already answered.
2. What kinds of help he is negotiating for specifically?
3. Is his fleet large enough that he can crack one of the Core World orbital defensive lines without relying on outside help?