StapMyVitals

I'm sure there was an errata or Warhammer Community post about factions like Devoted of Sigmar and Phoenix Temples where it said that any faction that didn't have allegiance abilities in the General's Handbook or a battletome just follow the rules of their Grand Alliance, so for example a Gutbuster allegiance army can use Ironguts as battleline but otherwise can only use the command traits and artefacts that Destruction provides. I can't find it right now, but my reading of it would be that Eldritch Council, for instance, has an allegiance ability which is the same as Grand Alliance: Order, which can therefore be switched with a Free City allegiance. This would mean that they can use the Free City rules for whichever cities they're included in (Hammerhal, Hallowheart, and Tempest's Eye) but would need to find a battleline unit other than Swordmasters if they did that.

This might ruin your day a bit, but the Celestial Hurricanum doesn't actually have the WAR MACHINE keyword, and so doesn't benefit from the Greywater Fastness allegiance ability.

The army I'd probably build out of that list would be - Freeguild General on Griffon (Indomitable Command Trait). - Freeguild General (Writ of Dominion artefact) - 20 Freeguild Guard - 30 Crossbowmen - 10 Greatswords for 960 points. All three units would be in a Great Company, and ideally you'd use the Swordsmen as a screen for the Crossbowmen, who'd be doing the bulk of your damage. The Freeguild General should be right behind the Crossbowmen so that the 12" Writ of Dominion bubble can be used on them, and the Griffon should be as close as possible to the Greatswords so they get a 3+ save as well as the +1 to hit they get for being near a Free Peoples hero. If you can get the Swordsmen into that bubble too, even better. The important thing is to keep those Crossbowmen above 20 models, more than 3" from the enemy, and with "Hold the Line!" used on them, because then on the Writ of Dominion turn you've potentially spent 300 points on 60 2+/3+ attacks with -1 Rend on a 5+ to wound, which is pretty good by anyone's standards.

Combines with the re-roll rules on the first page of the core rules.

That looks like a really solid list according to my experience. I've never used the Celestant Prime, but I think he'd be a good answer to the Freeguild problem of a lack of Rend power and mortal wounds. If I wanted to make one alteration, it would probably be the addition of another Freeguild General for insurance's sake. You'll still have the points for an extra command point, and being able to put Hold the Line! on three units at once means it's unlikely you won't be able to keep your infantry running at peak efficiency, but you will have that backup if they manage to snipe one of your Generals. All the better if they both have a Stately War Banner. You should also consider a Free Cities allegiance. At the minimum, Hammerhal is an extra point of Bravery in the Battleshock phase, which is never a bad thing.

From what I've heard, the new Wyldwoods will do potential damage to non-Sylvaneth every turn, rather than if they run or charge, so it might be slightly less effective (though that's not taking into account a guaranteed wood). On the other hand, I can't believe I never considered the Living City before as a list. Sylvaneth and Stormcast do what they do but better, but it directly fixes a huge problem with Freeguild: mobility.

Does discussion about Devoted of Sigmar fit into this thread? I want to field an army of them with support from a Celestial Hurricanum and a Knight-Azyros. I noticed that the War Altar's rule "The Power of Faith" states that "Devoted of Sigmar units from your army do not need to take Battleshock tests if they are within 10" of a War Altar". I'm wondering if this means you can if you want to. Flagellants do a mortal wound on a 4+ to an enemy within 6" if they flee from Battleshock, which is actually pretty good considering how dire they are otherwise. I'd like to know how much pushback I'd get from an opponent if I used the War Altar to choose not to take Battleshock when no enemies are nearby, then ignore it and use the Order allegiance re-roll to take as much Battleshock damage as possible when enemies are nearby, and ideally sacrifice those Flagellants to cause mortal wounds.

Azyr doesn't have its own artefacts in Malign Sorcery, but otherwise it would be very canon-friendly to have Freeguild from Azyr. Free People's playstyle is very static for the most part (though they do have some cavalry). Some of their best troops and especially their command abilities mean they get bonuses for staying where they are, and they have a lot of aura-based strengths which overall means that a Freeguild army is at its best in a tight battle line, holding ground rather than trying to take it. In a game about mobility and taking objectives, that can certainly have its problems, but it also has its benefits in that any enemy trying to wrest control of an objective from you is in for a hard time. Also their shooting might be some of the best in the game, so you can, if not take objectives from a distance, at least deny them.

I'd join the Flagellants into a unit of 40. They thrive on taking casualties, so you may as well be able to do that and fight back. If I'm being honest, I don't see where the synergy in this list is. Don't get me wrong, I like the idea of a fun Free Cities-style list based on what you have/a desperate coalition against the Ruinous Powers; I just think that there's nothing much in this army that helps out the other elements and so it'll be a rag-tag mix of units. Don't expect the Flagellants to do much, would be my advice. I will say at least you have a lot of speed overall, and that's not to be sneezed at, since I won my last two games with Devoted of Sigmar by running to objectives and sitting on them. And Flagellants may be quite frankly awful, but 40 wounds for 260 points and a 6+ invulnerable save near the War Altar is still a lot to chew through for anyone.

Freeguild/Free People have a lot of bonuses based on staying where they are, effects that trigger when they get charged, the numbers to take a mortal wound or two without crying, and several ways of mitigating battleshock. They're made as a defensive line. Even as a non-battletome army, they have representation in tournaments.