Allow me to preface this blog by saying: I don't think Sabo is alive, I truly hope he isn't, and if somebody mentions Pell, I will personally kick a little kitten.
Now we all know the theories, Dragon rescued him and he became a revolutionary, he's alive and waiting for Luffy in the New World as a pirate, those cannon shots didn't kill him, bla bla bla. Nobody cares about those theories.
What I do want to talk about, is what would have happened had he survived and joined the Marines. Ye, let that sink in for a minute. Garp always wanted Ace and Luffy to join the Marines, and become proud and strong members of the corps, but that didn't exactly happen now, did it? Anyways, back to Sabo. If Sabo were a marine, he'd be the perfect counter part to Luffy, he'd be what Luffy had become had he followed Garps wish. He'd be the Mr. Hyde to Luffys Dr. Jekyll (In a non-murdery way hopefully), the Darth Vader to his Anakin Skywalker, the Moridin to his Rand Al'Thor, the Sasuke to his Naruto, the Light to his Ryuzaki, the Tyler Durden to his unnamed Narrator, the Phoenix to his Jean Grey, the...Oh nevermind, I'm running out of references now.
Sabo, Luffy and Ace were pretty much alike as kids, even though Sabo wasn't a D. Luffy and Ace grew up with pretty similar personalities, so we could presume that Sabo would too, but where would he stand within the Marines? Absolute Justice, Lazy Justice, or simply his own kind of justice? A reunion with Marine-Sabo would be a perfect opportunity for Oda to show what Luffy would have been like as a Marine, whether he'd have kept his care-free attitude, or if he'd have become completely disillusioned with the whole piracy culture and turned to absolute justice to fulfill his duty.
Sabo as a Marine, would even fulfill DPs mad cravings for all the straw hats to have a tragic past. What could be more tragic than your sworn brother, turning completely against you and everything you stand for? I think we all agree that Luffy could do with some more character development (couldn't all shonen heroes?), and there's hardly a better place for it than a bitter fight with your brother.
Now that was my theory on why it could be cool, now let's just hope it doesn't happen :)