re: What would be the 51st state if it came down to it?

Current U.S. territory? Puerto Rico

Splitting a U.S. state? Let’s split Cali up i to multiple states or reorganize the Southwestern states via watersheds to fix water access issues.

Annexing a foreign nation: we should annex the settler colonies of the Commonwealth. This would create new states out of Canada (likely just using the Canadian provinces), the UK (each member nation like Scotland and Wales would become a state), Australia, the Bahamas, and New Zealand. The British pound as well as Canadian and Australian dollars would be pegged to the U.S. dollar and freely exchanged. There would be free movement of peoples and a common goods market between the states. The Virgin Islands could even be combined into a single state. This would also set up a system for the other commonwealth nations like South Africa, India, Malaysia, Belize, and others to get improved trade relations or even eventually apply for statehood.

Australia: 5 states
New Zealand: 2
Canada: 9 states (probably would merge some of the maritimes together)
UK: 5 states
The Bahamas/Bermuda/Jamaica: 1
Puerto Rico/Virgin Islands: 1
Singapour: 1
DC: 1

This would add 50 senators. We would also increase the House of Representatives by 50% to 653 and reapportion. The constituent nations would be permitted to maintain their national assemblies rather than be forced to actually dissolve into smaller federated states, but the state setup would determine their apportionment for Congress and electoral votes.

Outside of the English speaking settler colonies:
1. Overthrowing the Castros and taking over Cuba.
2. Giving the Philippines Statehood (something that was seriously considered in the 20th century).
3. Annexing Taiwan.