I think chapter specific box upgrades is well within the team of realism. Wasn’t it black Templar’s some years ago that had an upgrade book or I’m I thinking of something else. If you had a few chest plates that fit the current intercessors back on the torso, a few shine guards, then the usually weapons, heads, shoulder pads . And toss in some fur Pelts because why not.
Because without conversion work, a chestplate that fits back plate 42 on the Intercessor sprue only fits on backplate 42. It doesn't fit 10, 18, 26, or 34. It doesn't fit Hellblasters back plate 21, 29, 38, 46, or 55. It certainly doesn't fit any of the Phobos or Gravis armors. It doesn't even take into account the Assault Intercessors, because we don't have the multiparts for the kit yet.
So that really limits the usefulness of chest plates on an upgrade sprue.
Greaves are going to be the same - they fit a leg pose.
Back plates with pelts are going to be exactly the same way, except they are limited to specific legs and breastplates (unless you model them to slip onto any backplate, but you still have to try and make sure the pelt doesn't strike legs in any leg pose, otherwise it isn't useful, and that would really limit the flow/pose capability of the pelt).
It's the nature of the more expressive poses on the Primaris.
So if you are having to make three breastplates for Intercessors, three for Hellblasters, two for Reivers, two for Incursors/Infiltrators, two for Aggressors, and then leg replacements, and back plate replacements, all for unique poses in other kits - you've started getting to the point where you might as well just make a new kit, because you're basically already making that many core replacements.
Even arms crossing the body gripping two hands on a weapon are going to be limited to certain poses in the Primaris line, as would anything with cables or ammo belts.
Single arms holding weapons, helms, pauldrons, talismans, belt hanging items, etc., are really the only upgrades that are safe for just about any pose.
Folks really need to stop thinking that the Black Templars are somehow a cautionary tale for any of the Big Four - they aren't. They got one Codex, and it wasn't even the Edition they were the "poster boys" on the cover of the Edition start box. I know people like to crow about their popularity, etc., but while they might have had a cult following, they were never that important to GW in comparison to the Big Four.
Black Templars could be afforded to lose most of their uniqueness, the Big Four can't by much.
Let's not start acting like the BT forum and thinking hope is lost for the Wolves just because GW hasn't started releasing Chapter-unique Primaris units. Don't go down the "grim darkness" rabbit hole just yet. If GW hasn't released more Wolves unique units by 10th/11th Edition, or whenever the classic Marines Wolves boxes disappear forever, then we can start talking about how bad GW has treated them.