I might be thinking about this wrong, but my question here is what would Japan have to offer the Allied powers in this case? Without their militaristic expansion, the Pacific Theatre is effectively a nonissue (discounting China for the moment). It'd simplify supporting the Soviets with Lend-Lease equipment (cancelling the Arctic Convoys and instead shipping everything to Vladivostok and west via the Trans-Siberian Railroad), but that's about all I can think of.
The China question is...you'd need a much more defined timeline and PoD to get any useful answers there. Does Japan still seize Manchuria and Korea? Does the West support the communists or the Nationalists? Either way, that throws a considerable wrench into western involvement with the Chinese Civil War (neither side was happy with occupied Manchuko, but an Allied Japanese occupation strains things between the western powers and the KMT). And then this whole scenario spirals off into alternate versions of the Chinese Civil War that either split the Soviet-Anglo alliance or creates a very weird position of both supporting and not supporting Communist forces in Asia simultaneously, which turns into a complete and total hairball of three-way dealing that I can't even begin to imagine.
Hell, if you play it right this could end up with an east-west alliance of Japan, the western Allied nations and KMT/Nationalist China against a Central Asian Communist bloc of the PLA and Soviet Union. And that has...interesting implications for the European war as well if the Soviets break off.