I'm not sure, actually. I'm pretty well read on US war plans and the Japanese decision-making process that led to their embracement of the Pearl Harbour plan, but I'm less well briefed on the details of the alternative and earlier plans they developed for the Southern Strike push. I know they existed, undoubtedly, but the specifics of what they entailed I'm less clear on outside of some very broad generalizations. Maybe
@BobTheBarbarian can offer some insight on this?
The default alternative to the Pearl Harbor strike was a Tsushima-esque fleet battle ("Kantai Kessen") in the Philippine Sea, which had been the standard OPLAN in the event of war with the US since 1907. That year a Japanese think-tank hypothesized about the then-remote possibility of a conflict with the United States and outlined a "worst case" scenario that played out as follows:
1. Hostilities begin over the Philippines (triggered by opposing 'manifest destinies,' competing spheres of influence, economic considerations, etc.)
2. Japan occupies the Philippines. The USA's primary war aim is to take back the Philippines. (In later versions of this document, America's goal is the unconditional surrender of Japan via blockade and bombardment.)
3. The US Asiatic Fleet avoids battle and escapes to the East.
4. The US Navy combines its Atlantic and Pacific Fleets and regroups for a counteroffensive ("Thruster Plan")
5. The Japanese Fleet is detected early.
6. The US Navy concentrates its superior forces and destroys the Japanese fleet.
7. The USA retakes the Philippines and begins attacks on Japanese commercial shipping.
Admiral Tetsutaro Sato, the main author, pointed out that should the Japanese Navy have been defeated mainland Japan would be helpless and its people would quickly starve. Sato estimated that if Tokyo Bay were blockaded and all shipping lanes cut, 1907 Japan would be forced to capitulate in two months. Thus, the fate of Japan rested firmly in the hands of the Navy and the outcome of its surface duel with the USN. (A fleet in being strategy, while useful for deterring a blockade, wouldn't help Japan in this case since the latter's objective would be the preservation of conquered territory). The behavior of the US opponent was predicted based on observations of annual maneuvers in the Caribbean, various intelligence channels, past military experience, and analysis of the relative strengths of the two countries at the time. As it turned out, the findings of Sato and his colleagues were more or less in-line with the precursor to America's "War Plan Orange," then in development at the same time.
Admiral Sato later opposed the 5:5:3 USN-RN-IJN tonnage limit established at the 1922 Washington Naval Treaty on the grounds that it would hurt Japan's chances in the above scenario (Sato preferred a ratio of 2:3 Jap-US). This was where the Japanese Navy's plans for an "8-8" fleet (8 battleships and 8 battlecruisers) and "8-8-8" fleet (8 slow battleships, 8 fast battleships, 8 battlecruisers) came from.
(Admiral Sato)
This basic outline hardly changed much throughout the 1920s and 30s, with some alterations to account for advances in technology. The last versions envisioned the US fleet being whittled down by nuisance attacks from destroyers, planes, and submarines on the way to the Philippines, shadowed the entire time by long range patrol aircraft (H6K Mavis). For their part, Yamamoto and his supporters argued that even with the IJN fighting in its own backyard, the effects of attrition from Japanese raids and the logistical burden of operating a fleet halfway around the world would not sufficiently weaken the Americans to guarantee a victory in the
Kantai Kessen, which he used as another pitch to sell his Pearl Harbor attack. Bearing in mind Japan's superiority in carrier aviation and the fact that the USN rejected the "Thruster" plan as suicide years ago makes this argument seem dubious in hindsight, but the specter of a Japan denuded of its Naval shield so close to home and at the immediate mercy of the devastating blockade predicted by Sato surely had a strong impact on both military and civilian leaders, including Hirohito.
Source on pre-WWI Japanese-American planning:
http://www.nids.mod.go.jp/english/publication/kiyo/pdf/bulletin_e2003_4.pdf