How about Tsarevich Aleksey dying of haemophilia-related injury before 1916, and Rasputin never entering the picture in first place (maybe the former could be the result of the latter)? Say, Nikolai II goes into deep depression, and after some persuasion by his advisors (who happen to be a bit saner than those he had in OTL) decides to abdicate, and to dedicate himself to his grief - especially with the concern that he may not be able to produce another male heir due to his and his wife's advancing age, and both of them being sufficiently imbalanced to sign the abdication papers pressed upon them. In his stead, he appoints Mihail to rule as Mihail II, who does his best to withdraw from war - maybe by that time a major Entente offensive could coincide with this, and Russia would be able to withdraw from the war without a peace as disastrous as Brest-Litovsk.
Give Russia a few years to stabilize, more or less, become somewhat isolationist, and work on internal issues, and by about 1930 or so, with focus on economic development, Russia could be considerably better off - with post-war economic boom, and much of its industry rapidly modernizing, it could benefit from it much more than it did in OTL. As long as Mihail II stays out of the world conflicts, and concentrates on internal problems, by 1930 Russia might start resembling Britain to an extent, at least politically (even though the commoners would still get only limited power). However, it would be stable enough, and would have a generation of people who do know limited democracy to an extent - therefore, a step to true full-scale democracy would be easier, say, by 1960s, at which point it continues developing its resources and territory, becoming a true superpower by late XXth century via not entering major conflicts, and touting itself as a shining example of democratization done right.