the thing is, a good major work doesnt look at an event, but looks at the historians views on the event.
e.g you wouldnt write about the assasination, who did it and details and stuff like that what you would do is examine the areas of debate in your topic, like who killed her? well Mr Jackass beleives it was an accident, but this other historian thinks this happened.
so you present the historical issues, like your essay title wouldnt be " the assasination of Monroe" it would be something more like " " Monroe murdered? - an exploration on the historians views of Marilyn Monroe's murder throughout time"
and then you could argue it may have been an accident etc. thats just as example though, i dont even know if there is conflicting views over her assasination because i know shit all about it.
What i did was i was interested in the Aztecs... so i researched them heaps, then i focused down to when they were defeated by the Spanish, i found an area of debate that is the girl called Malinche [ an Aztec] who helped the Spanish defeat the aztecs
so instead of doing something like " Aztecs attacked - an exploration of the seige of Tenochitlan by spanish forces"
i did "Malince - Villain or Venerate? - A study of how historians over time have viewed Dona Marina’s actions during the Spanish conquest of Mexico"
so its a simple process, just work out what your interested in, find an interesting event, find an area of debate with conflicting sides and present an arguement OR examine how interpretations of that event have changed over time.