Let's say that the devil flipped a coin. On heads, the boat that Alvida raided found the barrel containing Luffy, allowing him to live, and everything happened as it does in canon.
On tails, the barrel sank deep into the blue sea and Luffy was never heard from again.
Naturally, this creates an alternate timeline.
Right off the bat, I know that Zoro and Usopp are dead. Zoro would have been executed by Helmeppo, and Usopp would have been killed protecting Kaya, who would also have been killed. Whether or not Sanji lives when Don Krieg attacked is up to debate, but chances are he would have fought to protect his ship and would have eventually been defeated.
On a different note, there's Buggy. When he was first introduced, he and his minions were fairly competent. Buggy himself was very cruel, as well. However, after he was not just defeated, but flat out -humiliated- by Luffy, defeated by having his body parts tied up, forcing him to journey as nothing more than a cephalothorax for Enel knows how many weeks. After his initial introduction, he's been shown as both cowardly and iincompetent. His spirit and confidence were broken. However, despite that he was powerful enough to sail the grand line, something Don Krieg's crew could not do, and not only that, he was coming fairly close to escaping Impel Down. If his confidence was not broken, I'm almost 100% sure he would have arrived on Sabaody Archipelego as a supernova.
There's also the matter of Skypea and Alabasta. Nobody would have showed up to help save both of those places in the nick of time, unless for some reason Luffy's death allowed a person to get there in his place. So far, the only people who could fulfill this are those that Luffy defeated previously, which off the top of my head include Buggy and Don Krieg. Don Krieg isn't benevolent enough to help, though Buggy -might-. Chances are, Vivi's dead, Robin's dead, and Skypeia is destroyed.
This is all I can think of off the top of my head. Feel free to add on to this AU.