Could Puerto Rico become part of a US state? I realise it has long been proposed as a 51st state, but that seems unlikely. So how about this alternative?
This would require the consent of Congress and the accepting U.S. state, but Puerto Rico's consent would not be constitutionally required (although it would be prudent to obtain and probably would be obtained as a matter of custom and fairness).
New York States has floated the possibility of annexing Puerto Rico since it is the single most concentrated destination of Puerto Rican migration to the mainland historically and has a large and politically well organized Puerto Rican community and would favor the expansion of Democratic political party power that this annexation would entail.
Contiguity is less of a concern for Puerto Rico since it is an island in any case.
Florida would be another natural candidate, but its conservative/purple political makeup would probably oppose such an annexation even though geographically it would be a more natural annexing state.
In a similar vein, there have been proposals to annex all of the District of Columbia except the federal mall to Maryland subject to the same formal requirements.
Either annexation would be a compromise. It would give these territories full statehood status and full representation in the U.S. House but would deny Puerto Rico and the District of Columbia, respective, their own two seats in the U.S. Senate which would favor Democrats more than mere annexation would.
Pursuant to the New States Clause of the Constitution (Art IV, Sec 3, Cl 1),
New States may be admitted by the Congress into this Union; but no new States shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or Parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress.
There are two precedents: Nevada (1866 and 1867) and Missouri, states which expanded their territory by absorbing part of an adjacent territory. The expansion of Nevada was enacted by Congress on 5 May 1866, coming in two parts. A large strip of Utah Territory was added
as provided for and consented to in the constitution of the State of Nevada, all that territory and tract of land adjoining the present eastern boundary of the State of Nevada, and lying between the thirty-seventh and the forty-second degrees of north latitude and west of the thirty-seventh degree of longitude west of Washington, is hereby added to and made a part of the State of Ne vada.
The southern chunk that is now Clark county was added after it was approved by the Nevada legislature. This thing is, it still requires congressional approval.