Darktide 2 isn’t needed (VT2 was needed cause VT1 had mechanical limitations toward what they could have brought, careers, weapons, enemy pathing, enemy models…). They can continue what they are doing without needing a new game.
Not every 40k game has to revolve around Primarch and SM. It’s a strength of the game, taking it out to implement SM would be a shame.
You do realise that those are much less varied than what we have currently ? So they have way less potential. If they do a game with a SM as an archetype they either should do it with the Deathwatch (To be able to introduce more varied forms of background and specialisations) or only have 1 or 2 Archetypes (So have a Sister of Battle; Tech Priest; Primaris Psyker or Librarian; and a Space Marine)
Would it be cool ? Sure.
Would it be cooler than what we have ? No.
Would it be the coolest thing the could do for the next (40k) tide game ? Also no.
- Orks vs Tyranids: Playing as Ork Nobz (Melee centred); Flashgitz (Ranged centred); Weirdboy (Psyker like); Mekboy; and Killa Kan (Big Archetype)
- Ynnari: Taking member for the Eldar, Dark Eldar and Harlequin
I’m sure that the SM fans would be interested in the game, but there are enough SM games around for them. I’d rather see other factions be featured, like Xenos as playable (Not in Darktide, but their own naratively separated game)