At the least, they'll have to either replace the finecast kits, or remove the models from the codex.

I honestly don't see them removing Crypteks, Lords, Overlords, and all of the special characters from the book, so we'll see plastic kits for most/all of those.
But, seeing the trends, it could be that the number of named special characters gets cut back and a few of those kits get retired, and the special characters become Lords of War.
Also likely to see at least one new HQ piece, maybe the Silent King, if the rumors are accurate from the Exterminatus book.

I think it's a 50/50 shot as to the survival of the Flayed Ones. Great flavor and fluff, but really, really bad in play. Could easily see these go away to make room for a new unit, of which we'll get at least one.
Figure we'll get either a redo of the C'Tan Shards, or maybe some sort of new kit for them. Figure they'll have some version of them in the codex and the Finecast piece needs to be replaced.

I could be mistaken, but I think that everything else in the range is plastic already. So not much to replace.
But I do see them adding at least one brand new unit and maybe a new vehicle of some kind--maybe some Necron Anti-Air--to make sure we buy some kits along with the the Codex/Supplemental Book 1-2 punch our wallets will already be taking.

Emperor be praised, this will be after the holidays, and I'll have holiday cash to blow on this stuff.