Like almost every brazilian that studies history, i have a soft spot for Dom Pedro II, and i hate our Republica Velha.
However, it's kind of hard to see my country being a superpower or something in these lines. The problem, in the end, wasn't just the choice between a Monarchy and a "Republic" (I don't really consider that we had any really democratic government before 1945), the problem was (and is!) about corruption, about all those oligarchies that still rule in everything but in name a great portion of my country. However, it's likely that the republican Argentina x Brazilian Empire would make a interesting scenario, to say at least. A world power? Maybe yes, maybe not. Our problems wouldn't be solved in a few decades.
Pedro was going to die in a few years, the Paraguayan War and it's aftermath aged him a lot, i don't really think that Isabel would have been a really able ruler, and her husband, the Count of Eu, was hated. Maybe giving Isabel a different husband and making her really participate (she was the presumptive heir of the Empire, Pedro wasn't really going to marry again, he had his affairs but he wasn't his father) of the government after the Paraguayan War would have improved things.
Pedro, in the end, loved our country from the first day of his government until his last, but he didn't cared anymore about the Monarchy, he really wanted to resign. He could easily crushed the coup d'etat of Benjamin Constant and Deodoro, but he simply doesn't saw much of a point in doing this.
If Isabel really had a oportunity to help Pedro with the burdens of his government, maybe things would have been different, and almost certainly better than 40 years of "Republican government".
And Pedro had a lot of sympathies for republicanism, the man was really uncommon, but he wouldn't have liked the government that succeded him, not at all, he likely would have supported a "English monarchy", but in those times, it's was almost impossible to do such a thing. He was a tired old kind man, he just wished to die in peace, without all those burdens and political crises of his late-reign.