I think this is going to work like a tree.

You have your main class - Lancer

Then you will unlock a Job which will further define your role in a party. I do not think we will see new weapons, if there are new weapons we will see new classes as well.

Your Job is tied to your class. You cannot be a Dragoon in any class other than Lancer. You may be able to use some skills from Dragoon in other classes like we can currently, but you will not be a Lancer/Dragoon, you will be whatever class/job you are with some Dragoon skills.