Italy's main issue was its enmity with Austria-Hungary, Germany's main ally. That made Italy the "odd man out" in the so-called Triple Alliance with the other two.
Italy had joined (reluctantly) with Germany out of a fear of France. This occurred when France and Britain concluded an alliance that made Britain responsible for the mutual defense of the English Channel, and freed the French fleet to concentrate in the Mediterranean, possibly against Italy.
When World War I broke out, Italy found that it had nothing to fear from France (or Britain, or Russia for that matter). On the other hand, it would have a lot to fear from a victorious Austria Hungary, from which she had taken Lombardy and Venice in the 19th century (the former when allied with France). So when Britain and France offered Italy Tyrol and Trieste from Austria, Italy jumped at the bait and switched sides.
For Italy to realistically have remained in the triple alliance, she would have needed to have acceded to Austrian demands, which were essentially a handover of Lombardy and Venice back to Austria. Essentially the Austrians had nothing to offer the Italians that compared to the carve up the allies were able to offer .
For the Italians to have been induced to join or remain with the central powers, the Italians would have needed to cede large amounts of her territory, accept total domination by Austria, accept economic ruin from the blockade, the loss of most, if not all her overseas territories, and probable starvation.
Italy would most likely have faced internal collapse and surrender by the end of 19154, possibly 1916 at the latest. Only small contingents of troops would have been needed to hold in the French alps, and tuisia due to the terrain, though the Italians proved very adept at warfare in the mountains.
Its a matter of opinion, but I think the allied forces sent to Gallipoli would have been diverted to the conquest of the Italian colonies had this situation have arisen. in early 1915 there were 84000 allied troops reserved for the assault on the dardanelles, and a further 45000 were earmarked for the ill-fated offensive into mesoptamia. These figures do not include the defensive garrison troops that were called upon to defend the allied bases after the collapse of these offensives.
A further 480000 allied troops were added to the TO during 1915, and a further 200000 in 1916. These troops were essentially held back, in training for nearly three years until mid 1917 after the collapse of the early campaigns. Its a matter of opinion, but I cannot see the Italians being able to resist in places like Libya, Sardinia and even sicily for long under the strain of a near total blockade and outnumbered this badly. I seriously doubt this TO would degenerate in the same way as the western front did.
Neither can i see much help developing from the other central powers.