@Deicide:
Lots of flashback characters are great: Bellemere, Hiruluk, Tom, Norland, Saul… These characters are often awesome and do awesome things that make them special, but guess what: their roles are really limited; everything they do is made of awesome because they do very little in the story, but in a great way. Once the flashback ends, they won't return.
So far I see Sabo as another flashback character. Let's hope Oda has different plans for him.
Actually I feel exactly the other way around. But that might just be my personal mindset.
Mainly because of the build up of Sabo's parents forcing him to marry a royal princess, while the visit of a celestial Dragon is foreshadowed.
I'm absolutely not gonna jump on the "Sabo for Nakama!" bandwagon, but that constellation might serve for some future plot devices. Depending on what he did, for Luffy to stop calling him his brother, he might even be some kind of villain in the current timeline, a royal with political power, who turned bitter over not being able to make his dreams come true.
Of course this could go both ways: Every flashback needs to fill its quota of "dead childhood friends or mentors". A loss which explains what the character strives for, why they became what they are today. So Luffy talking of " the only brother I have" might be a simple indication that Sabo didn't make it.
But I think this flashback might serve as an exception of the exception since the main story line already showed the death of a character most dear to Luffy.