If Yamamoto isn't killed in the April 1943 attack all I see happening is Japan losing it's fleet faster. Seriously. And that's assuming he isn't simply removed from command. He only kept his job after Midway due to morale concerns and there were increasing efforts as Guadacanal dragged on to remove him from that post.
(Please note, I didn't say losing the war faster, just losing the fleet faster.)
Yamamoto has a overblown reputation. He was intelligent, glib, and quick with a memorable phrase, but he was also rather mediocre as a commander. Both the Japan and US gave him a lot of good press after the Pearl Harbor, Japan to praise it's new hero and the US to "explain" away the attack; i.e. he must be a genius to have beat us that badly. However, Yamamoto's subsequent war career in no way matches his "successful" Pearl plans.
Midway is an unmitigated disaster, so bad that the IJN lied to it's own government about how many carriers were lost. Yes, the US was lucky during the battle, just as Japan was lucky during the Pearl operation. Yamamoto's plans however frittered away Japan's various strengths at the time which allowed the US' subsequent luck even more scope to effect the battle. As bad as Midway was, the Guadacanal campaign and Yamamoto's actions during it as CinC of the Combined Fleet point to a very mediocre commander.
For decades, Japanese and IJN planning for a Pacific war against the US had centered around a series of attritive battles in which the USN forced it's way through Japan's defensive perimeter culminating in a decisive battle near the Philippines where a weakened US fleet would be beaten. At Guadacanal, the exact situation which Japan and the IJN had been planning for decades presented itself and both the IJN General Staff and Yamamoto failed to realize that.
Here's a Japanese outpost, here's the USN trying to capture it, and yet Yamamoto did not use the opportunity to significantly damage the USN. To be sure, the USN lost many more ships than the IJN during the campaign, but Japan fed her forces piecemeal into the fight rather than massing for a truly damaging blow and the man responsible for that decision was Yamamoto.
When you look at the man's career after Pearl, there are no successes whatsoever. He isn't responsible for the "Lunge to the South", that was planned years beforehand. He isn't responsible for the Indian Ocean raid either. The operation and campaign he is responsible for, Midway and Guadacanal, are disasters however.
If he wasn't killed in April of 1943 and if he retained command, he simply would have lost the fleet sooner than Philippine Sea and Leyte.
Bill