The United States is supposed to be a Republic rather than a nation with an imperial government such as own empires. One might find that communist governments own empires sometimes- China annexed Tibet and the former Soviet Union wrought power grabs wheresoever it could. In that case the Communist Party simply annexed the new lands and made them part of the nation. The enhanced nation had different classes of state units and representation.
The United States could have had an empire if it had kept the Philippines after winning it in a war from Spain. Instead that colony was gotten rid of- granted independence after just 36 years. ref https://en.wikipedia.org/wiki/Tydings%E2%80%93McDuffie_Act
What would end in the United States would be a Republican form of government. That could happen. Eliminating the electoral college would allow dopified densely populated urban proles under the daily hate supervision of party and media corporate marionettes to yes-vote every politically correct item. It would be called democracy, although in fact it would be some kind of corporatism-socialist nominalism under an elite cadre of plutocrat directors.
The most interesting fin de siè·cle trend I have noted is the movement toward outsourcing jobs offshore while neglecting national financial competence in management (23 trillion of public debt) and letting infrastructure rot. There are additional challenges; failure to reinforce public college education for all with free tuition in order to remain competitive internationally is rivaled by needs to secure boundaries against illegal entry, resurrect ecospheric health and so forth. Those are serious challenges yet not greater than successful nations have overcome in the past.
The general social abandonment of traditional moral values is a serious danger for that will weaken public competence regarding virtue in political management sand social relations. The threats to the United States arise from elites with global agendas, over-the-border illegal aliens by the millions, and the corrupted Democrat Party without substantively high-quality leaders.