All,
In terms of historical potential, probably the United States (obviously), Russia (if it could've gotten its shit together and industrialized), Japan (human-controlled in 1835 means a Russo- or Sino- Japanese war before the West, apart from the UK, is capable of lodging a protest) or the Republic of X (where X is a Spanish-speaking Bolivian state, but only for roughly twenty-thirty years).
Additionally, a human-controlled US will probably crush the CSA immediately, annex larger portions of Mexico, Cuba and the Caribbean sugar islands and make a move on parts of Canada, things that would've been (or were) much harder historically. A human-controlled Japan will carve into the crumbling Spanish Empire in lieu of the US and Germany. A human-controlled Colombia or Venezuela can follow Bolivar and Santander's plan of uniting the Republics and driving into Mexico, industrialize and invade a weakened CSA or Brazil at once.
With a human player, of course, Spain can hold onto its colonies and go ahead with the planned counterrevolution in the Americas, the United Kingdom can maintain control over the dominions and strengthen its position in India (and China), France can make the rapid jaunt away from its mercantile economy and strengthen the Rhine, Prussia or Austria can unite Germany ahistorically early (and bloodily), etc.
The OE probably has a good shot at making a rebound if it can ward off the Russians and British, China
could, conceivably, hold its own with human guidance, but it'll probably (and should) be a complete mess, one of the major Sultans or Princes in India would have an easy enough time throwing back the AI... And, if a human in Europe really wanted to straight-jacket the United States, supporting the CSA and Mexico with arms and dollars (assuming this is possible, and it should be) while making a play for California or Oregon is the easiest way. Depending on how similar Victoria is to EU, building a coalition and taking her dead-on, even in the latter half of the sixties, wouldn't be very difficult. Same deal with an independent Texas throwing spanners into the works by tackling the Confederacy and colonizing westward.
Personally, though, I think the Zulus show the most potential... As Monty Python so eruditely demonstrated.
