I know Sentinel was designed as a sub skill, but what im suggesting is a possible way to make it its own class. Plus by making it its own class it offers many benefits, and a new class never really used before in a MMO, imagine fighting with a tower shield I see it pretty interesting, and it offers new skills that can also be used by the other classes.

Benefits
-New shields
-New skills
-New guild
-Additional Benefits to users of shields that decide to go pass rank 25
-New class
-New tank option for those that don't feel the current option fit them
-easier to implant than a brand new class.
-etc