Are you asking if there was a level of threat from Japan past which the US would have taken action, even if Pearl Harbor hadn’t happened?

I'm not exactly sure I understand the question completely; are you asking if USA was planning to attack Japan?

Let's address the 'capturing more pacific islands' part. One important strategic element US had in this regard were the Philippines they acquired after the Spanish - American war. Whilst not a 'puppet' in the traditional meaning, this allowed the US to base their Asiatic fleet in Manila as a forward deterrent to any kind of hostile operation in western pacific, and keep a presence close to the asian mainland. Japanese were well aware of this, launching invasions of Batan island and Aparri practically in parallel to the Pearl Harbour attack.

If you're asking whether US had plans to pre-emptively capture any of the pacific island Japan held or got as war reparations from Germany after WW1 ( Kwajalein, Truk, Palau, Saipan, etc ), not directly. These islands were addressed by the Washington and London Naval Treaties in 1922 and 1936; Japan as a signatory obliged not to reinforce the said islands furthermore ( but they ignored that and reinforced them anyway, especially Truk ) The US did make certain plans in case of war which were the genesis of the later 'frog-leaping' or 'island-hopping' campaign that was set in the War Plan Orange, and we'll get to that soon.

In any case, US had a series of so-called 'rainbow' plans, most prominent of them being War Plan Orange - a contingency plan for the war with the Empire of Japan, first started in 1920.

It would be important to mention the political context here - after the Russo-japanese war in 1905, with japanese interests in Liaotung secured, the rivalry between japanese Navy and Army wasn't waning, and Navy required an external enemy to focus on. A series of anti-japanese protests on the Western Coast, alongside with US's mere presence made it a viable choice, and relations between Japan and US only made political tensions between two powers escalate from there.

So the US was very aware of Japan's naval potential and planned accordingly - whereas War Plan Orange was built on a Mahanian Fleet-in-being doctrine that would show itself as obsolete with the consolidation of the aircraft carrier and aviation technology (not to mention the destruction of USN's capital ships at Pearl Harbour) certain parts of it were still useful, mostly the aforementioned gradual advance 'island-hopping' strategy.