I can see a number of challenges in terms of codex design for introducing Primaris that are peculiar to Grey Knights. So far, GW has shied away from producing Primaris units that are direct replacements for First-Born units. Grey Knight units are much less diverse than those of other chapters, so there seem to be fewer niches into which Primaris units could be inserted, without the new units ending up looking out of keeping with the existing design. The basic Primaris GK troop would be a Tacticus-armoured marine with a Cawl-pattern storm-bolter (R 30” AP -1) and a Nemesis force-weapon: lose either the wrist-mounted gun or the NFW and you’ve got an Intercessor with silver paint. However, you’ve now got a Strike Squad with 2W and 2A: except we’ve already got that, with the bonus of a 2+/5++ save. We also have to face the numbers problem: a Primaris GK army is likely to have even fewer models on the tabletop than it does now.
Some ideas that might be welcome: Inceptors with twin assault psycannons or psilencers; a fire-support Squad with an anti-tank variant of the psycannon; Aggressors with twin incinerators and a heavy daemon-hammer.
I really like the idea of Inceptors with psycannons. I kind of want to model that up now.