so long as your definition of empire doesn't specify a particular form of government, then i would propose this:
the Revolution doesn't change, in fact, nothing changes until the early 1800's with Jacksonian Democrats:
the compromise of 1820 is actually enforced (i.e. no Missouri Compromise). this leads to greater tension between the north and south earlier on. When Texas leaves the Mexican Empire we take to the Mexican War with much greater intent (in OTL this war was only meant for the South to gain more land and thus more "slave states" in Congress, because the comp. of 1820 forbid anymore land north of the 36(?) parallel to hold slaves). now in OTL the US conquered everything as far south as Nicaragua. it was because we essentially felt bad for doing this that we only took half that land. in this ATL there is no Missouri Comp. and thus no Bloody Kansas, (both events that acted as both boiling points and release valves). With internal strife running high and no hope for the south to gain land in the north, the same states push to take all of the conquered land. the Mexicans are not happy about this and there is a lot of rebellious actions that force the US to adopt a standing army for purposes of occupation. The standing army also creates a new sense of unity between the states and much of the tension is focused on the new territories. There is now a new racial demographic in a society that, at the time, cared very much about race as status. it will take at least a full generation to quell Mexico Territory. with the Mexican Demography now in the American society there is a buffer between the white and black race. As the Mexicans start moving north looking for work in the 1850's, farmers and shop owners who can"t afford slaves take them on in a renewed indentured servitude. this will slowly erode the slave labor, and with the pressure from the north, slavery becomes a rarity, however never abolished. i believe that by the 1870's things will calm down, just in time for the Spanish to challenge the Monroe Doctrine in Cuba. the US will go to war and, as in OTL, win. with this initial victory, Manifest Destiny will be renewed with a look south and, combined with the Monroe Doctrine, will take on a moral imperative; America will take to the islands. this will of course, create renewed hostilities with Britain. A war will start in the northern border with Canada and in the Atlantic with the Royal Navy. America will reorient the army to Canada and will trust the Marines and the Carribean Armada to hold the Islands. England will be strained of resources and manpower due to the onset of WW1. Canada will fall to the U.S. and Germany will win the war in Europe because America will not come to France' due to the XYZ conflict and its status of ally with Britain. Super status thus ensues and the American Empire is established as a republic whose colonies are all eligible for statehood. Germany will see America as a distant rival due to its continued distrust of European powers. due to the level of technology at the time, a cold war is avoided. Germany and the U.S. make a treaty that says the two nations will keep to their respective sides of the Atlantic thus creating a weak alliance held together by trade and mutual political distrust. Because Germany now controls all of Europe, and the Atlantic Wall treaty is in place, the former colonies and puppet nations in South America begin to fall under the control of Brazil. the U.S. now has a new power to look out for as both look to Central America, and especially the Isthmus of Panama. a treaty is made that both empires will contribute to the canal project. the Shared Colony of Panama is established as a means of mutual control over the canal. both Brazil and the U.S. have a type of congress whose majority is rotated every 2 years to govern that territory. the Pacific and Asia are now the only Wild Cards in the eyes of the Western Powers.