Fair, current expansion level, and you unlock some cosmetic/mounts/whatever is good enough, but the point would be that you are to get to 70 right now to start WWI, you are required to finish the campaign, unless you want to grind out.
Currently, you get to 70 by doing 1/3 of the campaign. Doesn’t matter so much the gear and/or level you finish, but that they slow down the experience and make it hard enough to make it feel like its own game.
Each expansion could feel like a solo/co-op RPG game that teaches you the class, lore and mechanics of the game