The mera mera no mi itself was some very subtle foreshadowing. We knew someone was going to get it and that the recipient would be very meaningful to Luffy. There really isn't a more suitable person to possess it.
While I agree that the weight of his dead brothers' dreams on his shoulders adds depth to his character, Sabo coming back gives Luffy something that he thought he lost forever - his dream of surpassing his brothers on his way to become Pirate King.
With Luffy's personality, he would never accept that he is truly the Pirate King if he never determined whether or not he was stronger than Ace. With Ace dead, he'd never know for sure if he was stronger in the end. Sabo solves this problem. We learned in the flashback that Sabo and Ace were virtually equal to one another in fighting ability. If Sabo eats the mera mera no mi, then he is a perfect stand-in for Ace (who was at one point a partial replacement for Sabo). Without a doubt Luffy will once again declare his rivalry with Sabo before the arc is over.
That said, I'll only truly be satisfied with the return of Sabo if Oda adequately explains why Sabo didn't try to save Ace.
I'm okay with Sabo not getting in touch with them sooner. If he was with Dragon who never went home to visit his son, it's perfectly reasonable that Sabo was never given the opportunity to come out of the shadows. I wouldn't be surprised if Sabo was a Loguetown with Dragon (probably standing next to Bartolomeo during the execution. haha)