In the 1920s and 1930s versions of WP Orange, before
In the 1920s and 1930s versions of WP Orange, before the RAINBOW plans, the USN's detailed plans for mobilization of the US Merchant Marine included liner conversions.
From the 1920s onward, the large fast liners built (or rebuilt) with USG funds (first by the USSB, then by the US Maritime Commission) had various earmarks for mobilization; at times, no less than a dozen large liners (including the 55,000-ton SS Leviathan, commissioned in 1911 as the German liner Vaterland and used under the US flag as a troopship in WW I and then converted to a liner - essentially a near-total rebuild and under the US flag until the mid-1930s, and capable of 28 knots burning oil). Unfortunately, she was completely uneconomic to run, and was scrapped (in the UK) in 1938...
Others included the modern (~1930) sisters Washington and Manhattan; California, Pennsylvania, and Virginia; and Malolo (half-sister), Lurline, Monterey, and Mariposa.
All the 1930s era ships were built with some military features as potential troop transports, and with at least some basic planning for carrier conversions; these were designated as XCVs, and the seven smaller ships would have approximated Ranger/Wasp in tonnage and would have carried a smaller but balanced air group of 18 fighters, 18 scout/dive bombers, and 18 torpedo bombers. Manhattan and Washington were larger (~22,000 tons vs. ~17,000-18,000) and faster (22 knots vs 18 knots) and, if converted, might have approached a Yorktown in size and a Ranger/Wasp air group (36 VF, 36 VSB).
In the event of a Pacific War that broke out before the expiration of the Washington treaty, they would have been very useful ships as carrier conversions; as it was, given the passge of the Vinson-Trammel acts and the need for shipyard resources for the Essexes (and then the Independence conversions) and especially the Two Ocean war and the needs of BOLERO and (as envisaged) ROUNDUP, they were more useful as troopers - these ships (along with USS West Point/SS America) are why the British 18th Division got to Singapore, for example.
In 1940-41, the USN also looked at Lafayette/ex-Normandie and the John Ericcson/ex-Kungsholm, but again, using them as troopers made more sense.
The need for fast carriers was met by the 24 Essexes and the nine Independences; the need for auxiliary carriers was met by the CVEs, both the C3 and T3 hulls and the Kaiser P2s.
The British, who had more liners than anyone else in the 1930s and 1940s, actually managed one such conversion, the Pretoria Castle, which despite her size was not actually all that more impressive in terms of capabilities than the handful of British CVEs converted from cargo or passenger-cargo ships.
Going back farther, there was Argus and the first Campania, but they both illustrate the problems with trying to convert ships designed to carry people into ships designed to carry things...freighters and tankers were actually better bets for successful carrier conversions than liners, generally.
Best,