Norther Mexico would be at best the furthest south American expansion minus seizing strategically important islands for trade and use as coaling stations later. When Grant considered the annexation of Santo Domingo a major obstacle was that there were "too many niggers" and eventually the project failed.
-William Walker might succeed in convincing the US to take Nicaragua, and in doing so be might be able to take Costa Rica and/or Honduras. El Salvador was asking for US statehood at one point, and Guatemala might be amenable to US takeover following the rash of civil war there in the 1870s.
-There were US cries to annex all Mexico in 1848, but presuming we take only the bordering stated to OTL (and southern Baja) along with the Yucatan, there is still the chance for civil war in the 1860s and 1910s with a Pancho Villa parallel of some kind. If the US surrounds it I could see the balance of Mexico digested and turned into states.
-Panama follows when we try ot build the Canal as per OTL as the Nicaragua route would be too close to the Costa Rican border, they are then surrounded on two sides and either become a dependent state or voluntarily get annexed at some point.
-Hispanola has several opportunities to become a state between our interventions in DR and Haiti, Cuba also has opportunities to become a state at several different intervals
-Perhaps the US asks for Lend-Lease bases in the Caribbean at some point or considers buying out UK interests in other parts of the Caribbean?
-We mentioned largest possible annexation of the US, Albania was apparently enthralled with the idea for a short time while there were (some) calls to annex Japan following the war. Then again, if we create a colony/satellite out of a surviving (joke of) South Vietnam it might be considered another state.
-Phillippines might be a possibility for US statehood though not sure that the locals would be totally enthralled with the idea.