I don't think the Imperial Guard will be getting any new models. Just the book and then their data cards.
Don't get me wrong, they might splash a new kit or clam pack, but the current trend is existing factions are just getting books, and as quickly as possible. But, the horde of characters who were only in that one Dark Eldar book, I'll be surprised to see return. I do expect a new Vect model at some point, especially as the narrative continues. Vect has some serious issues with the new Ynnari faction, and also has to deal with the Daemonic invasion of the dark city following the gathering storm part 2.
Dark Eldar (and all non-codex books), will be getting some updates in Chapter Approved, so you could easily see some points tweaks in that book. Wont be a huge overall (probably just points changes), but that could be nice to tide folks over until the codex comes out.