Well, I'll put up the (highly unlikely) scenario in which I think this is actually achieved, because the details here are important.
First off, Japan is going to need to kick off the war with a "Pearl Harbor" style attack on a major concentration of the Entente's naval assets in the Western Pacific; a tricky thing due to the fact that sending out a large naval task force is going to be noticed, and staging an assault on a fleet in a major defended harbor without air power is going to be a bloody hard affair at best. Maybe if there's some co-ordination ahead of time you pull it off with Von Spee getting a "Pied Piper" trail of pursuing vessels and feigning an attempt to make a run for Tsingtao and having the Imperial Japanese Navy spring an ambush. This would help achieve local superiority for a limited time, as well stoke fears in the Pacific holdings of the Entente powers that could make them more skittish to re-direct forces from the region toward's Europe, tying down a greater number of troops (Largely ANZAC's and Raj forces), giving the Japanese enough wiggle room to conduct offensive operations and facilitating a knock-on effect I'll get to later.
On land, the Russian Pacific ports that connect the Trans-Siberian route to commerical traffic from across the Pacific need to be cut off. This would tighten the noose around Russian commerce one notch tighter (As the Germans cut off Baltic traffic and the Ottomans cut off Black Sea traffic) and isolate any attempts by Czarist troops to regain territory or hold garrisons from their logistical base, making any such attempt wither on the vine and removing a potential front for the Japanese. Ultimately, the Siberian front is likely to be a short duration one; likely with the end result being the heavy fortification of a key hub somewhere along the Trans-Siberian line to blunt and decimate any Russian attempts to push back, leaving Manchuria to safely fall under Japanese hegemony (Likely under the direct control of a backed Clique)
Another key factor is they're going to have to get a friendly government (or series of governments) to gain the plurality of power in China; I'd say your best bet is Anti-Monarchist Beijang generals, who you can back in a coup against Yuan once he tries to put himself on the throne, since the C.P.M is too far away and not very sympathetic towards Japanese hegemonic asperations on the mainland. Getting the organize military forces on their side, they gain a useful baton to point threateningly at the British and French possessions; even if they aren't actually going to seize them (Though they very well may attack the Treaty Ports), the sheer size would be enough of a threat to limit Entente desires to stage an offensive or supplies for one out of them. Combined with the troublesome expanses of the Pacific to the east, Tokyo then has a fairly secure perimeter behind which she can prepare for the next offensive stage.
In order to do much more than this, the knock-on effect of Britain not deploying her Asiatic forces West is going to have to occur; the Ottoman Suez Expedition hits an unprepared Egypt and is successful in seizing the Canal and sabotaging/demolishing the Locks; throwing a monkey wrench in Britain and France's lifeline to their Asiatic colonies (and possibly causing the Ottoman Jihad declaration to start to hold water if it triggers an uprising in Egypt). With the Entente route to the Pacific lengthened considerably, Japan's period of local superiority is extended, and the shock triggers some modest shift in American capital loans from being made available to Britain and France to Japan (Which is combined with German-style "funny money" bookkeeping policies to lengthen the fuse on the financial timebomb). At this point, the Japanese can, if they're smart, probably have enough material stored up for one major expedition against a colonial position: French Indochina, perhaps, though it'd probably not be able to total displace the French positions. Still, it might weaken them enough to get a Pro-CP uprising in certain regions and break effective French control over the region, with her remaining military forces bottled up in a few strongpoints.
If all these peices fall into place, the US stays out, and the CP wins a negotiated peace in Europe (Russia collapses slightly quicker, the Ottomans are in a far better position, greater unrest in the Entente colonies... you could get this by late 1917-early 1918 even with the limitation of other butterflies), than Japan gets a seat at the peace conference and makes notable if still fairly modest gains in Asia; Hong Kong and Macau (Goes to them, or maybe tossed to China as a bone), recognition of the seizure of Russian assets and Japanese commercial hegemony in the North, recognition any client regeimes set up on the mainland (Be they Chinese or rump "White" puppets in the Russian Far East, reparations, and a smattering of Pacific Islands. Post-war, the nation faces major economic problems due to the war debts to its citizens coming due and a lack of access to forgien credit combining to form a liquidity crisis: probably hitting farmers extra hard as cheap rice starts coming in from the mainland and the soldiers return to production.