Regardless of how Smart develops from here on out, it's widely acknowledged that he's an elite perimeter defender. Worst-case scenario he'll end up a Bruce Bowen or Tony Allen type that sticks around the league for years as a defensive specialist.
The problem with an analysis like this is that Marcus Smart is that other than physical size and defensive credentials, Marcus Smart is NOTHING like Tony Allen.
Tony Allen is (and always has been) an awful offensive player. He was never capable of dribbling a basketball, he could never pass to save his life, and he's never been even the slightest bit threatening outside of about 5 feet as a scorer.
Even as a rookie, Smart already showed flashes of his potential as a solid passer, ball handler and shooter.
That's the difference between:
a) A good starter who specialises at defense
b) A good role player who specialises at defense
Unlike many specialist players (like Allen) Smart already has the foundations there to be an 'all round' player. All he really has to do is to make modest (but consistent) improvements to his shooting, passing and ball handling each year, and that will be enough to ensure he eventually develops into a good starter.
I really think Andre Iguodala (despite playing a different position) is a good comparison. With the exception of his defense he doesn't do anything at an 'elite' level - he does however do everything 'well', and that's been good enough to make him a borderline All-Star for most of his career.
I can see Smart developing into a similar type of player.