Whoo, good question.
Soulblight Gravelords
Deathrattle: Some of these units kind of work and others kind of don't, but I'd like them all to be reworked in order to give them a shared mechanical theme. Maybe all the deathrattle units could get the skeletons new 'get back up' ability and wight kings could grant some bonus to the roll. Or maybe all deathrattle units could have an ability that triggers on 6's to hit like grave guard (counts as 2 hits for skeletons, mortal wound for grave guard, mortal wound but only on the charge for black knights) and wight kings could grant a re-roll to attacks. Or maybe both, with foot wight kings having a +1 get back up CA and mounted wight kings having a reroll hits CA. I'd also like to see a bodyguard unit in soulblight, and Grave Guard / Black Knights seem to me like the most natural choice for that, so maybe that's something they could do in addition to or instead of some of their current gimmicks?
Apart from Deathrattle, I'm almost completely happy with the Soulblight Gravelord lineup overall. Some extra rend would be nice (either an extra pip on blood knights, or a +1 rend on target vampire unit in the vamp lore, or a -1 save debuff in the necro lore), but I don't think it's critical. I'd maybe like to see some of the ulfenkarn units go away (the bat lady & young Radakar can just be a vampire lord, the necromancer can just be a necromancer, the skeleton guy can just be a wight king), while I'd like to see new generic versions of others (gravekeeper, zombie ogres, lesser vamp infantry, swarms - whether as a unit or an endless spell). Speaking of, I'd like to see Soulblight Endless Spells. And models for the gravesites.
But all in all SBGL is a strong and flavorful book and most of its warscrolls are in a good enough place, which is honestly impressive given how many there are.
I'd like to see some subfaction changes - maybe a set of bloodline powers per faction as a unique enhancement for vampire heroes. Maybe change legion of blood to be less about deathrattle and more about neferata's spy network (redeploy friendly units, force enemy units into reserve, etc). Maybe change legion of night to be less about ambushing and more about necromancers & death magic (following mannfred's lore as a skilled necromancer, and developing the narrative by having carstinia take in refugees from the legion of sacrament). Vyrkos could then take up the outflanking gimmick as a sort of 'encircling wolf pack' theme since their current mechanics, while not bad, feel kind of arbitrary. Alternatively leave lon more or less as is, and introduce a new necrarch style deathmagic subfaction (the new underworlds guy could provide a basic template there).
OBR
Morghast Archai: Morghast Harbingers are relatively ok right now, but the Archai's slightly improved ward vs. mortals really doesn't compare to the Harbingers' 3d6" charge. They just don't have a unique roll. I'd like to see some differentiation from Harbingers in their stats - maybe -1 movement for +1 save - and a special rule that better fits their role - maybe re-roll ward failed saves for themselves and nearby Ossiarch heroes? Maybe give archai the bodyguard rule for monster heroes, and restrict the immortis guard to bodyguarding for non-monster heroes only? Or maybe a more aggressive take on a bodyguard, by buffing their attacks while within a certain range of a friendly OBR general? I don't know. But right now harbingers have a role in the army and archai just don't.
Vokmortian: right now he's mostly just a worse Soulmason, and he should probably be more & better than that. In particular some sort of tally or tithe rule would be cool.
Soulreaper: could use a bit of extra something. Maybe a rule for collecting souls when they kill models in melee, then spending them to buff their own casting rolls or those of other mortisans on the table? Something to give them a bit more flavor than 'cheapish hero that can kind of fight, that is also barely a wizard', because for just that the reaper is completely eclipsed by miir kainan.
Immortis Guard: I think their warscroll is mostly fine, does what they're supposed to do more or less. But their points cost could really stand to come down a fair bit. If archai are turned into a bodyguard unit, immortis might need something else to differentiate them.
Otherwise OBR don't need fixes to existing warscrolls so much as they need new units/warscrolls entirely to expand their range & fill in some gaps (ranged infantry, monster - mounted & otherwise, hero versions of the monstrous infantry), and revised faction rules to better mesh with 3rd edition core rules.
Nighthaunt & FEC
I'm really not familiar enough with these factions to comment, especially with the big changes to many/most Nighthaunt units coming in their new battletome. That said, it's at least nice to see that the devs are willing to make significant warscroll changes. I think that's a positive move even if the changes they get don't turn out to be what they needed.