This is the single greatest reddit comment ever and I’m saving this post for posterity.
Minis beyond the box expansions…
byu/forlorn_bandersnatch inBlackstonefortress
It looks like you’ve listed all of the ones that I know about.
There are 3 army-of-one characters: deathwatch watch master, eversor assassin, and solitaire
5 retinue characters: Jokaero weaponsmith, (2) Broggan brothers, Skarburn zapdakka, and Penitent 707
2 new enemy types: master of possession, greater possessed
The models that GW used for each of the ‘Extra’ characters:
The Jokaero weaponsmith, Deathwatch watch master, eversor assassin, and Solitaire are all as-is models from their own stand alone kits for 40k.
Penitent 707 is one of the three Arco-flagellants from the Sisters of Battle limited edition box set, which are different from the models in the stand alone Arco-flagellant box set. That being said, any Arco-flagellant model should do – it’s not like any of them have a number tag that says 707 anyway.
Skarburn Zapdakka is just a vanilla flash git from the Flash Gitz box set. You can make him using parts 04, 24, 10, 130, 138, 15, 142, 145, 28, 119, 106, 107, 84, 82, 127, 94, 95, 64, 58, 59, 49, 42, 40 from the Flash Gitz sprues. Or, just use any old Flash Git model.
The Broggan brother can both be made from the 40k Bullgryns box set, with one Ogryn left over that can probably be converted to a Chaos Ogryn (since the Traitor Command set is now out of print).
The Master of Possession and Greater Possessed were in the Shadowspear 40k box set, and are now in the Chaos Space Marine Start Collecting! box set (along with a bunch of extra Chaos Space Marines). Other good models for the master of possession include the Chaos Space Marine Dark Apostle and Chaos Space Marine Sorcerer. Good (but expensive) substitutes for the greater possessed are the Forgeworld Gal Vorbak from the Horus Heresy line.
FYI:
The old plastic chaos cultists from the Dark Vengeance 40k box set work well as extra chaos cultist. Some of those models can even pass as traitor guard (the models wearing guard-like tunics), and one looks like a dead ringer for a traitor commissar. Those models are all somewhat hard to get, but they are always listed on eBay. A small subset of the Dark Vengeance cultist are sold in their own 40k box set. Corpse Grinder cultists from Necromunda are another good source for Cultist models, especially Firebrands (except for the fact that you’ll have to scrounge up a flamer from your bits box).
The Plague Marines from the No Respite expansion were originally from something called Warhammer Conquest. Models with similar designs can be found in a couple of different 40k box sets. Extra Poxwalker models can be found from the 40k Dark Imperium box set (check eBay), and well as their one stand along box set.
