Every adult citizen should have representation in the Senate, House, and Electoral College.
The state structure of the Republic means that residents of territories and other non-states do not enjoy the franchise. This amendment to the constitution would require each territory to "team up" with a state, by mutual consent, for the purposes of presidential electors, senators, and congressional districts (and for those purposes only). These pairings could be reset every census-initiated redistricting.
For instance, Puerto Rico could join with New York, giving New York "extra" representation in the House and Electoral College due to its technically higher population. Puerto Ricans could then vote in the New York senatorial races and primaries, and there would be a representative in the House technically from New York but whose district included San Juan. DC might join Maryland, or Virginia, or even, if it wanted, Utah (if Utah agreed).
Important to note that in this scheme these "pairings" are strictly for voting purposes; these territories would not share laws (other than, maybe, voting laws), jurisdiction, or other legal or policy structures.
Lots of details to work out, but provides a way to give every US citizen full representation without breaking the state model or making every territory a state.
This small change solves a major philosophical problem with American democracy.