Which is why I'd probably play as an Apothecary or Techmarine for the most part.

Librarian or Devastator depending on their skills and playstyles. Probably both.

Devastator Librarian!

I will definitely be wielding a librarian, if I acquire a rogue hair I would probably default to an assault marine. Though I tend to play extremes and go with some unbelievably slow heavy weapon option. Though it truly does fall down to what powers a librarian will have. I see him becoming some form of buff machine in a full game instead of being a glorified mobile artillery piece of psychic powers. It will just come down to the powers.

Im running into all battles

i will be tactical marine but i hope they add the imperial guards so i can join the stormtroopers

I must say i would play all roles depending on what my brothers need. I will say if tech-marine is playable will he get to have/use those lil turrets or some kind of machine stuff? All in all I'd play tech-marine/ devastator and Tactical will be my classes and if terminator are playable them too.

Devastator for me. There's nothing better than seeing greenskin's or xeno's exploding in rain of bullets.

Grey Knights! They're all Tactical Assault Devastator Librarians, anyway!
If you think I'm kidding, look at the Dark Heresy supplements that gave rules for playing them. Outright absurd, and I don't think Ward even got his hands on those.

I'm leaning towards the Devestator class atm. I've always enjoyed larger weapons and anti-tank roles so depending on loadout I might even play Tech-marine.