We ARE NOT sure about anything until it actually happens. There has been leaked art of 3e style repackaging of some Gravelord units, BUT that doesn't necessarily mean they're coming with a battletome, nor does it mean the repackaging is happening soon. That sort of art can easily be done a year or more in advance. The current Soulblight Gravelords battletome was developed & tested alongside 3rd edition. 3rd edition branding had likely already been decided, so it's entirely possible that the art for 3rd edition style gravelord boxes has been sitting around in GW's system waiting to be leaked and or implemented since gravelords release even happened, so honestly even seeing that art now doesn't necessarily even mean that we're likely to be getting even just the repackaging any time soon.
There was a big rumor pile that said FEC and OBR are the spring death battletomes, as would be expected, but that a revised soulblight battletome will come in the fall. That rumor pile now seems to have some support making it seem somewhat credible, but the things it seems to have correctly predicted could have been lucky guesses, and even if that rumor was based on accurate information at the time, that information could also be out of date. Maybe obr and fec were intended for spring, but some production or development issue pushed one of them back and soulblight were moved up into the spring slot since their 3rd edition update doesn't really require much and might have been ready to go for a while. Or maybe obr and fec come out as expected in the spring, and soulblight don't show up at all - maybe because they got delayed for some reason, maybe because the rumors were wrong and there never was an update planned for them this year. Yes 3e style box artwork has leaked for some of their units, but again that box art could easily have been floating around GW's file systems since before 3e's release. It's a strong sign that we could be seeing soulblight this year, but it's not proof of that, and certainly isn't proof that soublight are one of the spring books specifically.
We just don't know.
To reiterate: the current strongest rumors are:
late Q2:
- OBR 3e book with FOMO/pity hero - a new debuff-oriented mortisan - and no other new kits
- FEC 3e book with FOMO/pity hero - a ghoul shaman / court mage - plus a dual kit of medium ghouls half way between little ghouls and horrors with one melee build and one ranged build.
Late Q3:
- Soulblight 3e book (don't expect major changes as the current book was already designed for 3e and is doing well according to GW's metawatch metrics) with new necromancer model and new dual kit grave guard / skeletal crossbow unit
....
Again, though, while that is the current state of the rumors, it IS just rumors so far. We could yet see all or some or none of this prove to be true. I'm honestly kind of hoping these rumors prove to be false, since those are absolutely not the releases I want to see for these factions. I dearly want that long hoped for Mortek Archer unit for OBR, especially in this current GHB season, and if they were to be stuck with only a fomo hero then I'd much rather see an infantry liege - which the army currently lacks - rather than another small wizard when there's already three generic units and two named units competing for that role. And while FEC does need any new units they can get, IMO what they need most are models for the courtier heroes that currently are stuck with resin or that have no kits at all, plus some sort of named heroes to give them some presence in the overall game narrative, which they currently painfully lack. And the current necromancer model is fantastic - they even already fixed the sprue so it no longer comes packaged with a square base. If they were going to replace a an existing hero it should be the wight king on foot who is still pre-modeled on an on-sprue square base. And why replace existing models at all when the hero soulblight players need and deserve is vampire lord on barded nightmare? Then again, new grave guard are needed and crossbones sound neat and cool, so if these rumors prove true there's at least one release I'll still be excited for.
So yeah, I'm not attached to these rumors because I /like/ them, they're just currently the most credible.
But the only thing confirmed is that GW currently are planning to release two 3e death battletomes in the latter half of Q2 this year, and that's far enough out yet that circumstances could still cause one or both of those planned releases to be delayed. nothing else is confirmed - not what factions they are, not what (if any) new models they'll be released with. Anything is still possible - though given how packed the year is both in terms of the sheer quantity of releases and how significant some of the expected releases already are (world eaters, 40k 10th edition, fully re-imagined cities of sigmar), the odds are heavily against any sort of entirely new death faction that would require a full new model range from scratch. That's the only possibility I feel comfortable ruling out completely.
(the claw is from the Khorne dude on all fours)