My guess for BoC is an O&G style release i.e. no new plastics at all, a couple of new resin characters (one named, one generic). I get the distinct impression GW have labeled them as an unpopular faction (why else would AoS willingly drop them from their range rather than keeping them as a dual system faction like S2D is in all but name). Hopefully I'm wrong though.
If I were being more optimistic, I could see GW releasing a plastic Tuskgor/Razorgor Chariot kit that can also construct a character on foot and, if you decide to go with Tuskgors, can be used to assemble Razorgors as a standalone unit. That replaces an old resin kit, gives official support to one that is currently unsupported, and gets rid of the awful Razorgor mini, plus gives an extra character option.
I didn't we'll get a plastic unit, but if I'm wish listing I'll say that if like to see Centigors get plastic sculpts (also Gors, but that ain't going to happen).
As for WoC, I think they're decently likely to get some plastic releases because I think they're viewed as a fairly popular faction (of the "evil" factions perhaps the most popular). I don't think we'll see Asavar Kul, I think he'll come later in a campaign book. I also doubt they'll recreate in plastic anything which has an AoS counterpart (Lord on Daemonic Mount for example), though I could see resin versions that fit TOW bases better.
For plastic, I think a paired character kit with a Chaos Steeds and the ability to build a battle standard seems likely (hopefully a Champion and Wizard, both who can either ride the Steed or be on foot, and the Champion can be given a banner) as that covers an obvious gap in the range. We'll get at least one named character (probably in resin), but I could see the Arcane Journal containing five (one for each mark) with more than one getting a mini at this stage.
I could see a Chaos Dragon kit: it's be a popular enough lot amongst collectors that it could justify the cost.
I kinda doubt we'll get any plastic unit kits though. Chaos is very well served by AoS and whilst officially the two shall never meet, I think there's a tacit understanding that people will but AoS stuff to use in their armies anyway. Maybe Forsaken will be given a facelift?
Of course they might introduce entirely new stuff in the Arcane Journal, and whilst so far they've expect that to be handled with conversions, they might buck that trend and release a kit. I think WoC are far more likely to get a plastic unit kit than BoC.
Of course this is all speculation. I think all that we can fairly safely say is we'll get resin characters for both 😄