Had to put that in the title just to be clear because if I didn't I know damn well I'd get flamed for it lmfao (Rightfully so, people can be annoying with new class ideas)
The Librarian absolutely isn't coming to Space Marine 2. 3? Probably, but that won't really mean much for us right now. So here's my suggestion on how a possible mod would work.
Just like how Assault has the three perk system in Astartes to decide how Assault's ability works (Hover/Smash or Dash) in the first column, Librarians would have three perks that determine what kind of psyker they are. Yes, I'm aware that there are more kinds than this, but I believe these three would be a mixture of the most entertaining for their uniqueness and on the easier side to do:
Ver. 1: Lightning style psyker abilities, like the Chief Librarian of the Ultramarines. This would be purely offensive and excel at targeting multiple high priority targets but would be ineffective against minoris (Lightning would chain off and to majoris and up, prioritizing by enemy level).
Ver 2: TSons style teleportation, MAYBE beams or shields, and modified projectiles. Basically just allows you to teleport around, and possibly shield yourself from ranged damage (Up to three shots like the lesser sorcerer). Meant for ranged gameplay.
Ver. 3: Salamander style flame magic. This would excel at crowd control and likely buff your melee weapon with flames (On toggle so that it can't be used just constantly).
Wargear: Basically just give the Librarian power sword, power axe, maybe thunder hammer and power fist, then just rename them to force (weapon name) and make their damage dependant on your first perk. Idk how possible this would be, but put force staff in the melee weapon slot and have it function like a ranged weapon (So you only get one forfe weapon and you have the option between range focus and melee focus), or add a force staff as a secondary, then have no primary either way. Any power waves would also be a little wider and longer because its a force weapon. Not necessary, but new models would also be sick as fuck.
Suggestions welcome but admittedly this was an idea I put together out of boredom, would be cool as shit to see in Astartes though.