Well... the Ottomans are in for a world of trouble when they join. Without a Central-Powers Bulgaria, the Berlin-Baghdad railway is under even more jeopardy now. And Germany is not going to be able to send aid to the Ottomans except through Romania (and Romania may not allow due to fear of Bulgaria and Russia jointly killing them). If Romania does join though, it is a continuous front for Russia in Europe, and a front in the Caucasus. So that will be tough for them to fully cover.
Now that Italy is safely in the CP, the Isonzo Front never is a thing, and the troops Austria and Germany kept there can be sent to other fronts. Will this make a difference in the front against Serbia-Montenegro-Bulgaria-Greece? Hard to say. And with a solid bloc in the Balkans, the Entente can threaten Austria as well as the Ottomans. And while a million German troops could defeat the 4 Balkan minors, a million Ottoman or Austrian troops is a much nearer thing. And Germany will need their troops against France and Russia.
Also, there is a front for Italy and France. This is probably becoming the next Isonzo Front. Germany and Austria will be forced to send troops there to hold for Italy, and they can't afford it...
Now Japan in the Far East can have a front with Russia, and it will distract Russia, but since it's Siberia, Russia can trade a lot of territory for time.
Expanding on this:
Bulgaria in the beginning of the war IRL had around 600,000 soldiers mobilized and 960 artillery guns. Serbia had around 250-360 thousand soldiers initially mobilized. IDK for Greece but let's say ~200-300k. Montenegro had 50k initially. That's more than a million troops mobilized for the war.
Now how many will be needed to hold the front with Austria, Ottomans, and (potentially) Romania? Let's say that the numbers are the same as IRL for the Serbian Front. IDK how many will be needed for the front with the Ottomans (depends whether they're doing an offensive push or a defensive). You guys are better than me at this, so I will let you figure out. As for the front with Romania, let's say it's the same as IRL Bulgarian troops on Romania's borders, maybe just a couple thousand more.
IDK was Italy's naval capabilities good at this time? I mean, they're incompetent for sure, but was their navy any good? Because if their navy is a threat, they could defeat the Greek Navy alongside the Ottomans and pose a potential naval invasion threat, but IDK for sure about that.
Now I presume that there will be a similar amount of troops that Russia has on the borders of Austria-Hungary and Germany. IDK how many more is needed to cover for Romania. Probably the same number of soldiers on the front with the Ottomans.
Now, will the Russian Empire be able to hold a continuous front? Doubt it. They already had trouble doing that IRL, if German army capability does not change, then the result does not change either. Austria's Galician forts are very much vulnerable thanks to Alfred Redl's sabotage, and they'd no doubt lose troops trying to hold Galicia. Austria's best chances here are retreating from Galicia and trying to hold in the Carpathian Mountains. But in any case, any advance into Galicia will be threatened by German movement into Congress Poland, so even if Russia makes some initial gains, they will probably be forced to let go of them to avoid encirclement. If they refuse to, and bite and hold, they will lose a LOT of troops to encirclement, but I am assuming Russia is not that stupid.
Now I think that with Bulgaria an Entente member, the Berlin-Baghdad Railway is under severe vulnerability, and Germany will not be able to get the oil they need. Will it hurt them a whole lot? Don't know for sure. You guys can figure that out for me.
I expect the Caucasus Front to go as bad for the Ottomans as IRL. This is bad for the Central Powers of course, but IDK if Germany can bail them out here. But perhaps with Britain remaining neutral they could send some more troops elsewhere? The Ottomans will be hard-pressed to keep Constantinople. I see them potentially losing it, which gives Russia access to the Med. Sea, giving them supplies they need. But could a joint Ottoman-Italian Fleet stop supplies from reaching the Russians? Hard to say.
Now, the Western Front. Assuming that the Shlieffen Plan goes same as IRL and Italy is in the war, France has to deal with an almost continuous front, only stopped by Switzerland. Now the First Battle of the Marne. Maybe, if Italy proves to be a large enough threat (or really, more of a distraction), France will be forced to send more troops to the Italian Front. This might (and I say a very big might) be able to distract France enough that they don't send nearly as much troops to the Marne, and with the British remaining neutral, perhaps it leads to a potential German victory (or a phyrric French victory)? This is a really big IF, and I don't know if this would really happen.
If Japan joins later in the war fully mobilized, and Russia is kept unawares to Japan's true intention, I could see a Pearl Harbor-esque event happening to Vladivostok, giving Japan a naval advantage early in the war, and stopping Russian trade in the Far East. Also, if Russia is kept unaware, Japan could surprise attack the Russian Army, taking thousands of prisoners and causing chaos in the Russian Organization (the Russian Army was not exactly the best in professionalism) until the front stabilizes. I could see Japan taking much of Korea, Kamchatka, and the Russian Far Eastern Ports if this surprise attack works. Japan's Navy will most likely be fighting the French Navy based in Indochina, and I don't know who'd win these clashes. Perhaps, if the French Indochinese Fleet is weakened enough to become a nonfactor, Japan could try naval invasions, but that is also a really big IF.
But these are just my few grains of salt. I'm not an expert in military affairs or wars, and I will not pretend to be, so just an FYI. What do you guys think?