Charlemagne, Louis XIV and Bolivar are interesting but you would have to come up with a really specific question to narrow your focus - if your focus is too broad it is very easy to drown in all of the information available. Napoleon can be tricky, but by comparing him to Hitler might make it easier.
Some more ideas...
- Did Robin Hood exist?
- How has the Holy Grail been portrayed throughout history? (Arthurian legends are really fun to do)
- The Princes in the Tower (this is not a common topic, but is really interesting and is still a controversial case today. It's about the 'murders' of Edward V and his brother, Richard of Shrewsbury)
- The impact and legacy of Eleanor of Aquitaine
- The impact and legacy of Richard the Lionheart
- The abdication of Louis-Phillipe, last king of France
- The death of Mad King Ludwig of Bavaria
- Did Noah's Ark exist?
- The division of the House of Habsburg and the legacies of the Austrian and Spanish Houses
Remember, don't cross the boundaries of HSC topics for Ancient and Modern, and make sure that your question is specific.
Hope this helps, good luck!