Here's the rough outline of my TL, since Adam wanted to see it:
-The Turkish defense of Plevna crumbles earlier and the Russians seize Constantinople before the British can intervene. The end result is a much larger Bulgaria, with Constantinople and the Straits as a Russian Hong Kong.
-Alexander II's assassination is butterflied away and he creates a successive constitutional monarchy, although the nobility remain strong in the rural areas (because the land reforms left them with the forests and the like, which they could charge the peasants for.
-Russia and Germany fought a very nasty war around the time of OTL's WWI that led to the creation of an independent Poland as a buffer state. If I want to wax apocalyptic, perhaps I'll have A-H disintegrate first and everyone grabs a piece, then realizes they're next if they keep this up.
-War between Britain and Russia breaks out over Persia, with the Japanese and the rump Ottoman Empire jumping in on the British side.
-The Russians basically bribe the US with valuable economic concessions to put the screws to the Japanese economically. They launch an attack on the Phillippines, bringing the US into the war against the Japanese. The US and Britain are not at war at first, but the Irish in America HATE Britain and will likely cause trouble, and support anti-British political candidates.
-The British get nukes first and use them against the Russians and loan them to the Japanese, who use them against the Americans. In TTL, Pearl Harbor gets nuked. The US declares war on Britain.
-At some point, American forces liberate Ireland. The British nuke the forts defending St. Petersburg from attack by sea and conduct landings along the Baltic coast.
I think the later parts of the TL need work, as the British would strip everything bare to defend Ireland against an American landing, which would be seen coming from far away.
Basically, the premise is that some 19th Century trends--America's friendliness with Russia and dislike of Britain, the "Great Game" between Britain and Russia in Central Asia, and the British-Japanese alliance--all continue.