Italy and Romania would both be fairly simple to make switch sides, as both of them were basically staying neutral until one side or the other was in a strong enough position and/or made them a good enough offer.
Past Italy and Romania, the other neutral that is easiest to bring to the CP side is Sweden. It had a pro-German King and Army, even if the Social Democratic majority in the Legislature was pacifiist. There is a juicy potential PoD at the onset of the war, however: the gung-ho Commander of the Russian Baltic Fleet, acting on his own initiative, almost issued an ultimatum to the Swedish Fleet to vacate their bases and do not return. OTL he was discovered and blocked by his superiors in the nick of time. Had he gone, it is very likely that a naval battle would have ensued. Social Democratic doctrine of the time heartily endorsed defense warfare, so ti is exceedingly likely that in the face of this act of aggression the Swedish Diet would heed the King's advice and declare war on Russia in 1914.
Differently from Italy, a CP Sweden (or Romania) is not enough by itself to change the strategic balance of the war decisively, but it most likely does accelerate the collapse of Russia from Winter 1917 to Summer/Fall 1916. This may, or may not, change the outcome of the war, but most probably does. If Russia falls sooner, Germany will not be motivated to engage in unrestricted submarine warfare, so Wilson lacks a casus belli and USA stays neutral. Germany being able to redeploy most of her forces to France 3-6 months before, with less cumulative exaustion from a shorter blockde, could quite possibly have broken the back of the French manpower camel, with no US aid coming, and caused a strategic breakthrough.
In terms of relative tilt to the strategic balance in favor of the CPs, Italy ranks (well) before Sweden, and Sweden (significantly) before Romania. Italy by itself has the manpower, a good army, and the strategic position advantage to cause a fatal crisis in either France or Russia, either very soon in 1914 or over 1-2 years in 1915-16. Sweden has much less manpower, but has a decent army and good strategic position (threat to St. Petersburg) to accelerate Russian collapse significantly. Romania has not the manpower, a barely decent army (but terrible generalship), and the main effect of her switch is that Serbia falls in 1914, the CP can penetrate Western Ukraine too in 1915, and they can apply the effort they needed to crush Romania in late 1916 for added pressure to Russia, too. Again, this probably accelerates Russian collapse to 1916. No Entente USA, likely French collapse in 1917.
Of course, a CP USA would have been a massive tilt to the balance: suddenly the British blockade of Germany becomes a questionable thing and the vast majority of the British Army needs to be redeployed to canada. Adieu, France.
Of course, it would have needed a different president (Germanophile TR wins Republican nomination in 1912 and landslides Wilson
) and/or a rather more skillful German diplomacy. Of course, the latter also makes rather likely that Britain stays neutral or is allied to the CPs.