That people have something to work for, easy as that. I would lock them behind a questline that can be started once you reach max level with your previous toon. So that it's not something people can start at day 1 of the expansion.
Unlocking something is a goal players can work towards. Oh and I am not talking about something as badly done as unlocking flying or something gated behind reputation. Make it a good, long and challenging questline that doesn't rely on weekly grinds. I'd say a micro class is worth the hassle.
I mean you don't need those restrictions I made up with armor classes. But those micro classes - if they're a thing - shouldn't be available just by purchasing the new expansion. They should be a reward for playing the expansion.