The Puertorican POV
I was born in New York but I'm living in the island right now and pretty much the feeling is split between statehood and the continuance of the colonial status. The independence movement is only followed by a small group and is pretty much dead.
I agree with Grey Wolf in that the best way to have a independent Puerto Rico has to be a pre-1898 POD. Except for the violent proto-fascist Nationalist Party terrorist attacks in the late 40's-early 50's(the attack to Congress and to President Truman for example) and the early 1980's small scale terrorist attacks by the Macheteros group(the destruction of every single one National Guard fighter plane on the island in the early 1980's for example. That raid was used as an example of a well planned operation in my counterterrorist class on the Marine Corps in the early 1980's) the island has been very calm. But in the majority of the cases the population see themselves as Puertoricans firsts and them as Americans.
The invasion of Puerto Rico was an afterthought of the Spanish-American War. Maybe peace before american forces landed on the island so no legal rights for annexation? I know I'm pushing it
Also PR was a British colony for 3 months on 1598 after the capture of the city of San Juan by a british force but cholera forced the British invaders to withdraw. A independent member of the Commonwealth, perhaps?