I would try to study the effects of individual humans, Great-Man vs Historical Inevitability style, by knocking out statesmen of a particular period. Hitler is a cliche, whom I'd nonetheless start with; but I'd follow up by seeing what happens if you kill Chamberlain, Churchill, Roosevelt, Stalin... and work my way down to the likes of Turing and Doenitz. Do you still get France overrun in six weeks? A resurgent German nationalism? A defiant to-the-last-ditch mood in Britain? And so on.

Then I'd start on similar questions for the unification of Germany: Bismarck, Kaiser Wilhelm, Franz Josef, Marx, Napoleon III, and so forth. Then perhaps the Great War or the Cold War, or perhaps I'd be bored with recent history and go for something medieval instead - Harald wins at Stamford Bridge, perhaps. Or to maintain the remove-one-person style of the experiment, there's the three claimants to the British throne, one could kill Edgar the Confessor earlier, the Pope has a hand in it, there's the various dukes and other feudal lords in England... lots of fun to be had with this scenario!