Two scenarios that come to my mind, both can be considered the best case scenario for Central Powers:
1. In late 1914 and early 1915 various neutral powers decide to gang-up against the Entente and thus joins the CP:
Italy, Japan, Romania, Sweden, Denmark, Netherlands, Spain, China, Persia, and Greece, while Ottoman Empire and Bulgaria join the CP too as in OTL (I deem Norway, Belgium, and Portugal are too pro-British to include them).
United States still joins the Entente in the later stage, like in OTL.
2. Similar with the above,
Italy, Japan, Romania, Sweden, Denmark, Netherlands, Spain, China, Persia, and Greece joins the CP, but this time it's much worse: in later stage
United States joins the CP too. However,
nearly all Latin American countries (including Mexico, Brazil, Argentina) decide to throw their support to the Entente and join them, thanks to Anglo-French naval supremacy.
So...which scenario that has bigger chance to make a CP victory TL...?
(well, maybe this is too close to the ASB, but still...)
