I think I'm just as sure that Sabo is dead as Greg is sure that he's alive. I haven't ever in my million+ years of reading this series ever had a bigger BAAWWW moment than I did reading Sabo's letter to his brothers. The Letter provided so much closure. It would feel so cheap to me if it ended up as this:
Oda: "I know you guys are still really sad about Ace dying, so… I made a new brother for Luffy! He's just as cool and strong as Ace and he has a hat too! So please forget about that meaningless Ace guy 'cause we have Sabo now!"
If Sabo shows up in the present storyline, for me, it would be a cheap cop out. A way for Luffy to still have a cool older brother, even though the rescue attempt for Ace failed.
I really think it's going to be Luffy's family that brings him out of his current state, much like he's done for every one of them at some point. As awesome as a reunion with Sabo would be, I just can't see it. There are far too many things in the series that are stacked against Sabo's return that would have to be overcome.
The two biggest reasons that come to mind immediately:
1. Sabo not contacting either of his brothers for all these years.
2. Sabo didn't come to save Ace during the war.
I've heard all of the fan excuses for reasoning away those two issues and frankly they aren't good enough for me. Sabo certainly isn't the type of character that would let someone else control him so for me the whole "Dragon is keeping him from contacting anyone" is completely ridiculous, as is the amnesia routine. I just haven't heard anything that sounds remotely plausible as for why Sabo hadn't contacted his brothers, and why he didn't show up to the war.
After reading Sabo's letter we see just how strong his bond is with his brothers. Sabo's bond with Ace and Luffy is his treasure. I don't think that there is anything in the world that could keep him from them. Other than death...
I just read the raw again (high quality raw is up by the way) and have to agree that Ace's "ブッ殺してやる!!!" moment really makes the hairs on the back of my neck stand up. That entire scene is so expertly done. And now we have to wait another week to see the rest... Ugh.