What would be the impact if *both* Italy and the Ottoman Empire remained neutral throughout all WWI?
I'm of the view they both mattered and carried weight for their coalitions. Their absence would be felt.
As stated elsewhere I think the net bonus is to the allies - although a lot depends on what restrictions the Turks put on merchant shipping through the straits and international reaction to that. Italy not joining the conflict takes a good bit of pressure off the Austrians and also means a looser allied blockade of the CPs. However Ottomans neutrality means no fronts in Sinai, Iraq or the Caucasus Mts. It could well also means that Bulgaria stays neutral which would give a markedly stronger position for Serbia, especially if say some of the forces that OTL went to fight the Ottomans aid Serbia instead, probably via Montenegro, which has ports on the Adriatic. - This could also prompt clashes between the Austrian and western navies.
The big uncertainty would be what traffic is allowed through the straits however as that would probably be a war-winner for the allies. It was said that the Ottomans blocked allied merchant shipping a month before they actually joined the war but would they still do that if they decide to remain neutral? Or if say Russian grain or allied supplies are bining carried in neutral shipping say?
Also is one trigger to this that Goeben is either sunk by the allies or goes to the Adriatic rather than Constantinople?
No disaster at Gallipoli, but sadly that would butterfly away the bonds created by the ANZAC forces. Perhaps ANZAC day will be commemorated in another event.
I don’t think there’s anything sad about avoidance of the Gallipolli disaster. If New Zealand and Australia hate each other more and bond less over British plans screwing them over it’s a small price to pay.
Common experience taking bloody casualties on the western front might be a replacement experience for ANZAC bonding. Or perhaps later fighting against Japan if there ends up being a showdown against that country sometime.