I'm trying with a fresh start, summarising the things I have from a very trusted birdy. According to this source, the models released over the next weeks are:
"Glotkin" ... a named character, not a mount option for anybody, the biggest kit to be released (though not as big as a Stompa, as rumoured by others)
Nurgle (?) Chosen ... the uncertainty here is whether they are actually dedicated to Nurgle, or "only" very mutated
A new big mount ... for "multiple types of riders", but most likely to be WoC only
Named character in clampack ... no details here
The upcoming WD points to "Scions of Chaos" next week, which is most likely the first batch of these going on advance orders on the evening of October the 10th. I've been told the Chosen and the clampack character are the week #1 releases. With a bit of uncertainty: the "big mount" for week #2, and Glotkin plus book in week #3.
There is no box like the 40K "Stormclaw" one coming.
So I've been informed that the "Nurgle Blightkings" are infact the unit previously running as "Nurgle Chosen". And the clampack character seems to run by the name Gutrot Spume...
Regarding the WFB plaguelords:
Maggoth Lord - nurgle monster + rider set, new never before seen box allowing 3 options:
• Orghotts Daemonspew, Chaos Lord on Whippermaw.
• Bloab Rotspawned, Chaos Sorcerer on Bilespurter.
• Morbidex Twiceborn, nurgle champion on Tripletongue.