Let me start off by saying that I don't think game 3 will feature 4 chaos faction of the different Chaos gods at launch. Not only would that disappoint people who don't want to play as Chaos, it would also **** off those who bought the Warriors of Chaos DLC for their warriors to be launched for free on day 1 of game three. As such, I imagine the base game races for game 3 will feature Ogres, Chaos Dwarfs, Kislev and Daemons Undivided. And before you say it, yes, Daemons Undivided is a thing. That's how they were represented in their original 8th edition army book, and they didn't get separated into god specific armies until Age of Sigmar. As such, Daemons will likely start out as one united faction, and separate god specific armies will likely come as DLC if they're coming at all.
Also, as we should all expect at this point, units from each game 3 base race will be held for DLC in the form of Lord Packs. This means that it's highly likely that at least one god in game 3 will be represented in the form of a themed lord pack round them. This will probably disappoint the fans of said god or gods, but that's most likely what's going to happen, as only 2 Legendary Lords will be available for them at launch, meaning only two gods will have representatives in the form of Legendary Lords (and this is assuming Be'lakor isn't one of them.)
Given how much N'kari has been pushed in the teases in game 2, I think Slaanesh is safe. It'd be silly to not have Slaanesh full roster if the strongest Keeper of Secrets is going to play a massive role in the next game's story. I also think Tzeentch is a bit safe since Lords of Change are currently the only Greater Daemon in the game right now. Worse case scenario, Kairos Fateweaver might be a FLC lord since he would be relatively cheap to make given they have the current Lord of Change model to base him on. That leaves Khorne and Nurgle and, well, since Khorne is the more iconic and impactful of the two, I think Nurgle will be the DLC theme. His aesthetic is popular enough with fans and would make a great theme for a DLC with Kislev. No matter what happens, I don't think we should be surprised if fans of one god need to pay a bit extra to represent them in the game.