Jar Jar as the ultimate Sith? Hah. Well it's possible, but I doubt it was in any way considered during filming of the prequels.
What a lot of hardcore fans don't understand, is that they are probably more invested in the story than even Lucas is
![Stick Out Tongue :p :p](/styles/default/xenforo/vbSmilies/Normal/tongue.gif)
Whilst they may assume that Lucas has in his head an entire universe of characters and back-stories, the reality is more likely to be that things get thought up on a film-by-film basis.
For Lucas, it could even be something as simple as revenge
"Oh, you didn't like my Jar Jar character? Well now I'm going to make him the most powerful Sith in the universe. How do you like him now, buttholes?"