I would like to ask some experienced players what class would be better on new server Frostmourne. I am thinking about warrior/rogue. I would go for war but i know there will be planty of plate armor players (and lot of warriors). Maybe I am wrong.
Idk what makes you think that. Warr is weak without high armor pen and no SM. In general RDPS will probably be more favored because they can spread for mechanics better.
I suggest a flexible class as your first choice. Mages, Warlocks, Hunters and Rogues can only do damage and because of this you might find yourself getting bored or frustrated with longer waiting times. I would suggest a tanking or healing class as your main character because all of them also have a solid damage dealing specialization that you can work on when you get dual spec.