Coincidence... This week now I was thinking just about this question you raised
Well, some things we must have in mind while drafting a TL, if you focus on a particular nation/movement/continent:
= respect the rhythm of the technological, economic, social and political evolutions, lest we fall in the "Dominion of Draka" AH. No industrial revolution happens to an impoverished country in a decade, or such. Even Japan and Russia, with its extremely peculiar technological revolutions, had a series of pre-requisites that allowed for it, but we can't expect the same to happen in early 19th Century Portugal, for example.
= Try to imagine some really bad scenarios and episodes for your focus of interest. If we look at France during the 100 Years War, Italy during the 16th Century, China during the Mongol invasions, Russia during the Bolshevik revolution... History witnessed a series of cataclysmic disasters for each one of them, but a series of historical developments allowed for their phoenix-like "ressurgence" from the ashes.
= You raised a very good point regarding the string of good luck rulers and wars. This indeed happened historically (everything related to Napoleon seems like a French wank, let's be honest), which proves that it's an useful tool, we just must be careful to not abuse it
= Finally, the most aspect that helps you to be realistic is to study the most you can about the history of the particular nation/movement/continent you are focusing on. No one imagined England/Britain would become the paramount nation in the world in the Victorian Age if you keep things going on exactly as they were in the times Boudicca. There were centuries of historical developments and revolutions that allowed for one of the most egregious "Historical Wanks"

. You might replicate it in your TL, of course, but that takes a while and a serious effort.