What if the Italians did not enter into the first world war? I know this had been done before once or twice, but since thread necromancy has been frowned on lately....but given the other threat that inspired this one, why not?
There has to be a reason for the Italians to stay out, which probably means a decisive show of force by the Austro-Hungarians early in the war, so they don't seem like such easy pickings. This will create butterflies beyond just the Italians staying out, but it is crucial to make this idea work. The most likely POD is that the AH 2nd army stays in the Balkans (well, 3 corps and the attacked Honved divisions), as the 2nd army didn't arrive in time to help in Galicia anyway, but still was caught up in the retreat in September.
This gives AH the manpower to push down the Morava river valley and occupy Serbia all the way into the interior of the nation, depriving them of their economic heartland and the bulk of their population. Though Nish will still hold out in the initial onslaught, it provided only 80 shells a day for the artillery, very few rifles, and no machine guns. The Salonika port remains open though, which means some resupply is possible. This means that throughout August the Serbs are pushed back into the interior, by which time the 2nd army is pulled out entirely, allowing the remaining 5th and 6th armies to hold against the battered Serbs, who now lack their main recruiting areas and still have to hold down the rebellious, recently added territories to the south. Bulgaria is somewhat more likely to join in, but not guaranteed. This way, it looks like the AHs are able to win, but were forced to delay the coup-de-grace, thanks the the Galician situation. They don't lose nearly as much face to the Italians, and the Serbian Front can hold without the Serbs being able to launch the kind of attacks they were able to OTL.
So, Serbia is not beaten, but removed as a major threat to the AHs and the Italians are not as likely to consider the AHs as a push over. By no means does this mean the Italians won't attack, but they have had enough reason to give it a second thought now. The situation in Galicia is roughly the same as OTL, though the AHs have had a morale boost that was missing historically. Now the Germans can make their case properly for knocking Serbia out of the war, as the Serbs are now a much easier target. This will probably require German manpower thanks to the situation in the Carpathians, but that alone with guarantee Bulgarian help. So we now have an earlier Bulgarian entrance into the war, the Serbs are knocked out in March, the path is opened to Turkey, making the Gallipoli lands precarious, and the Salonika Front isn't opened.
At this point Italy would have to be cowed out of thinking about joining the war. The AHs are not a pushover and are only fighting on one front, completely unencumbered by commitments other than the Eastern Front. So Italy stays out of the war, just as Gorlice-Tarnow is starting.
What effects does this have on Italy and the war? No Italian Front frees up a lot of AH resources for use against Russia, meaning that the same burden on their economy and manpower doesn't exist. But then again, neither does the popular war that helped solidify AH will. However, when the Russian Front wraps up, the AHs have no more active fronts to fight on and can focus on demobilization/rebuilding their economy (minus the minor commitments to the Western, Middle Eastern, and perhaps occupations in Ukraine/Balkan Fronts).