1902 - Teddy Roosevelt decides to make Cuba and Puerto Rico territories of the US. Cuba becomes a state shortly thereafter while Puerto Rico follows in 1912
1903 - the Hay-Herran treaty fails in the Colombian Senate, leading Roosevelt to intervene in Panama, but he decides that the only way to ensure stability os direct control - the Panama Territory is born in 1904 and statehood comes in 1914.
1912 - Nicaragua is occupied by the United States as part of the ongoing Banana Wars and instead of releasing the nation to its own devices it is made into a territory in 1918
1915 - Pancho Villa wrecks havoc along the US-Mexican frontier and raids into US territory. In response the Wilson administration sends an expedition force into Mexico in 1916 that eventually results in the occupation of most of Mexico due to its instability under Carranza. US takes Sonora, Baja, and Chihuahua to protect its existing borders while making Veracruz a free port and stirring up nationalism in the Yucatan but not letting them form a true nation. Tamapulias, Nuevo Leon, and Coahila will eventually move into the Union of their own accord when their governors secede to form a new Republic of the Rio Grande prior to annexation as one state in 1921
1916 - US occupation of the Dominican Republic begins as the US ousts a corrupt government, but decides to unite the territory with occupied Haiti into one government of Hispanola which becomes a state plagued with guerilla warfare for three decades
1918 - Manila becomes the capital of the US Western Pacific Territory after the locals continue ongoing guerilla warfare. Although an Independence Party will exist even into the present day, this territory becomes a state in 1932 at the same time as Hawaii
1923 - After seven coups in three years, the US occupies Honduras in the name of fruit company interests, making the region a territory in 1925 and a state alongside in Nicaragua in 1928
1940 - The United States takes over Iceland and Greenland from Denmark, after the war they elect to stay with the US as a territory and a state respectively. Dominica also becomes an island pccupied as part of lend-lease that after the war votes to join the US as a state, which the UK permits on condition of the acceptance of statehood which comes in 1954
1976 - the Quebec vote on Independence results in a 53% vote "for" and the nascent nation rallies to the secessionist cause. As a result they become their own country in 1981, but the provinces of Nova Scotia, New Brunswick, Prince Edward Island, and Newfoundland apply for statehood after long negotiations with Ottowa. These become the two states of Atlantica (New Brunswick and Nova Scotia) and Newfoundland (PEI does not have enough people to become a state on its own).
1991 - British Columbia and Alberta vote to secede from Canada after they feel marginalized by Eastern Canada. As a result there is a long series of discussions that result in the full merger of the United States and the balance of Canada in 2010. Also in this year Albania, fresh from the brutality of the Cold War, asks to become part of the United States and is accepted.
1995 - Guyana elects a party dedicated to merger with the United States and its government asks for Union, which is achieved in 2010 alongside the Canadian merger
This makes for 21 new states not counting a potential Eastern Pacific Territory or something more ASB like Sicily or New Zealand joining the US. All of the above nations could easily have become States under the right or wrong circumstances and it might be plausible to have the rest of Mexico fall into US hands here. If someone would make a map it might illustrate the situation better