Note that Bladegheists will only fight in two ranks in the Crimson Doom procession, as the rule only applies to battleline. So it's a big boon to bladegheists in crimson doom and dreadscythes in quicksilver dead. Not as much of a boon for grieving legion or emerald host, though, as the rule would really only apply to spirit hosts (not really combatants), chainrasps (can already fight in two ranks due to small bases) and grimghasts (can already fight in two ranks thanks to reach). the formation to count as extra models will certainly help those processions in claiming objectives, though.
The big winners for nighthaunt really do seem to be crimson doom. Good for people who want to spam bladegheists over and over again. Personally I prefer a variety of units for purely aesthetic reasons, but if you had to spam one nighthaunt unit the bladegheists are the coolest looking & most dynamic. And red nighthaunts painted like swirls of splashing blood do look quite striking. GW should be able to move some extra 'blood for the blood god' at least.
For vamp counts, all of their relevant units can already attack in two ranks due to being small, though zombies counting as 3 models to claim objectives sounds impressive. As a loose sketch, maybe something like: Volga, Manfred, vamp lord, necromancer, 60 zombies, 40 zombies, 40 zombies, 5 blood knights, 10 dire wolves? Actually, wait, dire wolves can qualify as fancy vets, and actually do meaningfully benefit from fighting in two ranks now that their minimum squad size is 10. Maybe some sort of volga led vyrkos list with a couple units of 20 or even 30 dire wolves?
FEC... I don't know. ghouls can already attack in two ranks, horrors are bad regardless, and flayers afaik tend to run in 3s. Maybe reinforced flayer units could be a thing? And summoned units aren't part of battalions, either, so all the ghouls that archregents can bring to the table don't care about the formation bonus.
OBR... the formation to count as extra models is great for morteks, but they already fight in two ranks due to being adorably tiny, so that change mostly just means the things they fight are going to get more attacks back at them. Their only other battleline is cavalry, so unless we get a book or update in the next 12 months that makes stalkers battleline, this shouldn't be a particularly big game changer for them.
Yeah, my first reaction was that this would be a huge deal for death, but actually looking at the units it could apply to, taking account existing reach and base sizes, it seems the only undead armies that the previewed rules make a big difference for are Crimson Doom and Quicksilver Dead. I think in general this is going to turn out to be a bigger boon for our opponents.
