The general marine traits will be expanded to incorporate things that all marines would be privy to at different costs based on their specialization. For example: melee combat talents that are rank one talents for an assault marines would be available in the general marine talents section at rank 2. The cost of the talent would be increased according to the specialization of the players character to represent the time and challenge of incorporating training outside of their already demanding role.

IE: A tactical marine and a Librarian both want to take the swift attack trait at rank 2. The trait costs the Assault marine 500xp at rank 1 so the tactical marine can buy the talent for 750xp at rank 2 and the Librarian can take the talent for 1000xp at rank 2.

of course this does not include abilities from true specialists like the librarian or techmarine as an assault marine all of a sudden becoming a psyker would not make much sense.

The example doesn't make a lot of sense since the Librarian can normally buy Swift Attack at rank 3 and the Tactical Marine at rank 6, why would it be cheaper for the Tactical Marine?

I allow cross-training in my campaign, you can learn a skill or talent(within reason) from another member of the squad, but you have to be one rank above the skill or talent becomes availble for the trainer and it cost you double the exp cost. Certain talents, as you have said, can not be learned like psyker powers and machine traits specific to tech marines.