I'll go down the list:
Italy: Possible, but a bit difficult to pull off; the Italians wanted a lot more than Austria-Hungary was willing to give in order to join the CP. On the other hand, if the Central Powers start winning the war, Italy's terms are likely to go down; the Italians really wanted to be on the winning side.
Scandinavia: Denmark and Sweden were more friendly towards Germany, while Norway favored the British. All of them were pretty strongly neutral though, and would probably only be drawn in if the Entente struck first. Pulling them in is within the realm of possibility though; Sweden has the near-incident with Admiral Essen, and some of the Entente plans tossed around for trying break open the Baltic would probably force Denmark into the war.
Latin America: Unlikely, because the logistics of crossing the Atlantic and a hostile Britain means they would have no realistic hope of influencing the war's outcome. If they join in at all, it would be an 11th-hour "we want to be on the winning side" declaration of war.
Persia: Depending on one's view of the
Persian Campaign, it could be argued that the Persian government was a de facto member of the Central Powers OTL.
China and Japan: Possible, but very difficult. Germany's East Asian colonies were easy pickings that would be hard to resist, while the Entente had a far more entrenched position in the area. Also, China is not stable enough to face a major war; conflict with the Entente alone is probably too much for them to handle (especially if Japan is still part of the Entente). Japan also has it's long-standing alliance with Britain to consider.
Also, to toss another two nations into the mix, Romania goes into the same category as Italy; Romania had territorial desires against both the Entente and the Central Powers, and only joined when they thought the Entente was on the verge of winning.
The Dutch are another outside possibility; they were vaguely pro-German, and there were a few vague mentions of giving the Dutch Flanders post-war. However, the Germans were of the opinion that the Dutch were more useful as a neutral middleman for trade than they would be as a military ally, and the Dutch would be extremely hesitant to expose Indonesia to British attack.