My understanding is that some quite robust debates and conflicts between the IJA and IJN, inspired by the ‘victory disease’, at high command levels in Japan in February / March 1942 led to Operation FS as a compromise to extend Japan’s reach to Fiji and Samoa to cut off the air and to some extent sea lanes from America to Australia to prevent America establishing a base in Australia to strike at Japan’s ribbon defence from the south.
However, I’ve just found this: http://ibiblio.org/pha/monos/152/152app01.html
Top Secret
Combined Fleet Operation Order No 1 [1]
On board the flagship Nagato in Saeki Bay, on 5 November 1941
Commander in Chief of the Combined Fleet Yamamoto, Isoroku
Combined Fleet Order
The operations of the Combined Fleet against the United States, Britain and the Netherlands will be conducted in accordance with the separate volume.
Combined Fleet Top Secret Operational Order No 1 separate volume.
Operations of the Combined Fleet against the United States, Britain and the Netherlands.
[1] As the order is extensive in scope, details have been omitted and only the essential points are given, with some explanation.
The relevant part is
Section 3. Areas Designated for Occupation or Destruction
The following areas will be occupied or destroyed as soon as the operational situations permits:
a. Eastern New Guinea, New Britain, Fiji and Samoa areas
b. Aleutians and Midway areas
c. Andaman Islands area
d. Strategic points in the Australia area.
I’d be interested to know if there is a corresponding IJA and or Imperial Conference or other imperial / combined high command level war plan before the war began which identified Fiji and Samoa as potential targets.