Well, to start off with, I wasn’t a fan of the whole "Reason of State" quest. I dislike having to choose between blatantly condemning Roche to death or having a Nilfgaardian victory. The whole thing seemed a bit lazy. Another thing is that Iorveth could potentially fit in quite well with assassination plot, and although I initially dismissed his absence as being justified given that he’d have no role in the game, I’ve come round to thinking that he could actually fit quite well - if things were done differently.
Iorveth's provenance[]
First off, the assassination plot itself. I think they definitely could���ve included Iorveth here. Given Radovid's witch hunt of anything not strictly human, it’s only logical that the Scoia'tael would consider him an enemy and seek his removal. As Iorveth himself is said to be a shrewd and cunning leader, I think he would’ve realised that the North being in young Redanian king's hands would spell serious trouble for both the Scoia'tael and elves generally. Furthermore, if Iorveth's path was chosen in The Witcher 2, Vergen would’ve presumably fallen to Redania when they invaded Kaedwen and that whole area during the 1271-1272. Even if it had fallen to the Nilfgaardians instead, I’m sure Iorveth would’ve been fully aware that Radovid would have his sights set on the city and the non-human population within. Case in point, I’m sure Iorveth would want Radovid to be dead - probably more so than Foltest or any other king.
But how would this have anything to do with Dijkstra and his plot? Well, after Dijkstra is forced out of Redania at the end of the books (late 1260s), he travels with a major Scoia'tael officer and member of the Vrihedd Brigade: Isengrim Faoiltiarna. It appears that the two became unlikely friends. As a result, it's quite possible that Dijkstra has contacts among the Scoia'tael. After all, as he knows everything about everyone and has a number of spies working for him, it s plausible that he came into contact with Iorveth; if he has connections with everyone related to the Novigrad underworld, the Scoia'tael aren't that much of a stretch (in fact, there's also a Scoia'tael camp not far from the city). With Radovid and his witch hunters becoming a big threat to non-humans, Dijkstra certainly would have noticed this and could've then used these contacts to reach Iorveth and begin planning Radovid's assassination with him - just as he did with Roche.
Preparing the assassination[]
In terms of how the preparation would go, I can see Roche and Iorveth - albeit reluctantly - working on it together. By then, the two's paths had diverged and they both had something much bigger to worry about: Radovid. Roche was worried because the young king threatened an end to Temeria, and Iorveth would've wanted him dead for the reasons stated above - that Radovid wanted a fanatical crusade against non-humans (as the default war ending shows). Not only that, but Roche puts Temeria before all else and is professional enough not to be put off due to the presence of an enemy. Iorveth, meawhile, proves that he is willing to work with dh'oine if needed (Letho, Saskia, and optionally Geralt), so I can see him and Roche grudgingly agreeing to work together on this. In my opinion, Iorveth's presence in all this is plausible.
In terms of quests, Iorveth would need one just like Roche and Thaler, the other conspirators. Perhaps a non-violent mission where he and Geralt have to get some supplies to some Scoia'tael commandos, where Iorveth can mention why he's part of the plot, what happened to him after Loc Muinne, his opinion on the war and whatnot. The content for such a quest wouldn't be too important, I think; the important thing is that one exists for him. As for Kaer Morhen, he could also be a guest during the battle if his quest is completed. They could also have it so it's either him or Roche that go and not both, or maybe Iorveth wouldn't go if Letho is there, or Letho and Roche? Not massively important, that, but it'd still be nice if he there was an option for having Iorveth there as well.
Radovid's assassination and the aftermath[]
So, on to the assassination quest itself. Iorveth would obviously have to be in the warehouse with the others, but I think he should stay with Dijkstra instead of going with Roche, as numerous Scoia'tael commandos on the streets would raise suspicion. From there, the actual assassination and the run-up to it can be the same as it is in-game: Geralt lures Radovid into Novigrad, Roche and his men ambush them, Radovid is cornered and thereafter killed by Philippa. That's all fine, in my opinion. However, the crucial difference in this version would be that, instead of Geralt accompanying Roche immediately, he's asked to join him later on due to Roche having to arrange some political stuff with the others now that the deed has been done, which obviously wouldn't have much to do with Geralt. Oh, and also, Geralt would go to Dijkstra's bathhouse instead of the theatre, as there's more space and the surroundings are more enclosed. When Geralt gets there soon afterwards, the coup is already in action; he finds Roche's men locked in combat with Iorveth's elves, with no sign of the co-conspirators. He is then ambushed by a few commandos with their weapons drawn, at which point Geralt has to make his choice:
Option 1: Tell the elves that he's not with the Temerians and he wants no part in what's going on, has no quarrel with the elves, and that he'll let his co-conspirators sort everything out for themselves. At this point, Iorveth steps in and assures his men that Geralt will keep his word, before bidding the witcher farewell.
The cause of all this would be that Dijkstra rejected Roche's deal with the Temerians - just like he does in-game - along with Iorveth. However, Dijkstra and Iorveth secretly made their own deal: in return for Scoia'tael support in the war, Dijkstra agrees to make Upper Aedirn/Lormark a vassal state to Redania for non-humans to reside, with Vergen as its capital. However, the game-changing difference between this new state and Dol Blathanna is that the Scoia'tael would be welcome there, which gives Iorveth a reason to accept it. When this was revealed to Roche, conflict broke out, hence the fighting.
As a result, with this ending, Nilfgaard are defeated and Redania annexes the North with Dijkstra as its ruler. However, Upper Aedirn/Lormark, as promised, is given to non-humans as a vassal to Redania. Roche, meanwhile, is killed by Iorveth and his men inside the bathhouse, alongside Ves, Thaler and the other Temerian guerrillas.
Option 2: Tell the elves that he demands to know what's going on and that he won't let Roche come to any harm. Realising that Geralt isn't with them, the Scoia'tael commandos attack Geralt and are cut down, before Geralt finds Roche fighting with his men, surrounded by the Scoia'tael. Here, Geralt saves Roche's life and the two agree to hunt down and kill Dijkstra and Iorveth. Geralt goes after Dijkstra and, after fighting his way past a few of his men, kills the former spymaster. Simultaneously, Roche, Ves and the remaining guerillas go to find Iorveth. When they do, a skirmish breaks out in which Iorveth is slain.
This ending is the same as the in-game one, where the Northern Realms are leaderless, resulting in a NIlfgaardian victory and Temeria becoming a vassal state to Nilfgaard.