Land-Grant University. Cheaper than a normal university and provides +1 to each tile improvements yield. Some examples of land grant universities are: Cal Berkley, MIT, Michigan State U, Minnesota, Cornell, Wisconsin, Arizona.
Land Grant universities were also all designed to have Reserve Officer's Training Courses to train commanders for the US Army, so could provide a discount on forming new military units as well.
For Unique American Buildings, a short list:
Town Hall - Loyalty, Amenity bonuses. (This could be an alternative to the 'Meeting House' and more precise to America)
Department Store - major Gold, Amenity bonuses
Union Station - a railroad terminal used by all railroads serving a city: Trade, Amenity, Gold bonuses
Unique Improvement:
Stockade: a Fort built by US Military Engineers, can be built on unclaimed tiles within 4 tiles of tthe American border and turns the tile it's on and the surrounding adjacent tiles into American territory.
Possible Unique Units:
NOT the 'Minuteman' which was a mediocre militia outfit.
Legion of the United States - a Line Infantry unit with Scout-like vision and movement (the original Legion had line infantry, light infantry, light artillery and dragoons all in a regiment-sized unit, oe of the best Combined Arms units ever designed for rough country actions)
Mounted Rifles - a Line Infantry unit that moves like Cavalry (the first 'Kentucky' Mounted Rifles rode in loose formation to the battlefield, but used mostly mounted or dismounted firepower with muskets, rifles, shotguns, and/or pistols)
Yellow Legs - US Cavalry of the 19th century, no anti-mounted bonus applies when fighting them because they almost always fought dismounted.
Redlegs - US Artillery of the early Modern Era. Can fire twice in one turn (the Fire Direction Center invented by the US Army Artillery School in the late 1930s could direct an entire division's artillery to mass fire on a target within 3 minutes of spotting the target, and within 3 - 5 minutes after the guns finished firing, redirect them to another target - no other artillery in the world could come close to that performance)
FireFinder - US Rocket Artillery automatically attacks any opposing artillery unit that fires within range of the rocket artillery (the Fire Finder radar could pinpoint an enemy artillery position before the first round they fired had hit the ground, and the MLRS (M270) rocket artillery could put several thousand sub-munitions on that position within 2 minutes. In the first Gulf War, that combination wiped out over 70% of the Iraqi artillery)
Heavy Frigate - the US 44-gun frigates of the post-Revolution: they could out-fight even small shps of the line.
Monitor - coastal Ironclad with extremely high firepower
Essex Class Carrier - aircraft carrier, get two whenever you build one (the US built 27 Essex class fleet carriers in less than 4 years: it is still the largest single class of capital ships built since the 18th century and more aircraft carriers than the entire rest of the world combined managed to build in WWII)