France occurred to me too: we tend not to recall what a close-run thing it (repeatedly) was. Germany (like Italy) seems a bit well-trodden, but could be interesting with a non-Prussia (or non-Piedmont) focus. Austria/Austria-Hungary's always fascinating, and this period encompasses the whole 1804-1918 empire. Spain & Portugal would be a bit more "out there" and are quite a challenge, but they're potentially rewarding and less vast than Russia's rise at the other end of the continent. And of course there's always the familiar rise of representative government in Britain and national aspiration in Ireland as in Poland or the Balkan lands.

Further afield, it's the "crisis" of the Chinese empire - but how much of it was home-grown, how much exogenous? It's the period of British supremacy in India and the first stirrings of modern nationalism. For Latin America it's the era of independence, export-led development and the rise of US hemispheric power. In Africa it's the period from the end of the Atlantic slave trade to the start of the full-scale colonial scramble: how were they linked, and what happened in between? Globally the century sees perhaps thirtyfold growth in international trade and the rise of gold to monetary hegemony (and the first signs that it may not have been such a brilliant idea after all). And it's the great age of European industrial & population growth and emigration (the last however mostly from the 1840s) - though others too were on the move. And of course it sees the rise of secularism, science and challenges on class, race and gender, though these were all foreshadowed in the 18th century and sometimes the 17th.

A fascinating century, then, with considerable unity but in some respects marked by a new dynamism from around 1850, yet it's arguable that we didn't truly emerge from its shadow until a century later - but that's anther story lying outside the chronological range.