I totally agree, im of the mind that any new codex should have at least three new units as a way of justifying its cost to buy for the player and the work of the writer and artists, the three units could be three clam packs or three dual multipart kits, as you say though the other cult troops do need plastic more than khorne did, a truly themed kit for each of the other three gods in plastic would win over many a chaos fan. Its a no brainer, its as plain as the wart on nurgles ass crack.
It's hard to tell lately if the plastic treatment rumors pertain to 40k or Fantasy, as the Blightkings and Wrathmongers are basically God specific WoC Chosen, but didn't we hear some hulabaloo about Nurgle themed marines on newer 32mm bases? I recall that for some reason but maybe I am just pulling things out of the air.
Tzeentch has definitely come up more than once lately too, so I'd wager we have a good bet that he will get some plastic stuff in the future.