I'm a big fan of getting ahead of myself.
Right now, Only War has the following specializations: Weapon Specialist, Heavy Gunner, Medic, Operator, and Sergeant for Guardsman, and Commissar, Orgryn, Ratling, Sanctioned Psyker, Ministorum Priest, Mechanicus Enginseer, and Storm Trooper for Specialists. It's a fine list, all in all. Covers a lot of the lovely old Imperial Guard.
But looking to the future, what would you like to see added?
I'm a big fan of bringing back some of the Dark heresy careers as Specializations; the Adept and Arbitrator both have special places in my heart. Some kind of non-Ratling Sniper could be a lot of fun; perhaps build him for stealth and make it a Sniper or Assassin? Bringing in more abhumans might be nice, too, what with Squats being canon again and the interesting addition of Felinids. I'd love to see a game about pilots get added later, too.