I was just thinking about interesting fight scenes/stories that could be filler and have nothing to do with the plot of the story:
Some of the missions with Shikamaru's, Neji's, and Kiba's teams before they entered the 2nd chuunin exam. Since I'm biased to the Shikamaru three, I wouldn't mind seeing their teamwork in action that made Asuma recommend them as rookies to take the chuunin exam.
Speaking of teamwork, anyone think that the teamwork spiel that Kakashi made such a big deal out in the first episodes has been lacking? I want more team action!
I think it would be also interesting to see episodes with the legendary three working together before they split up and Orochimaru started being evil...although that could be considered a flashback rather than filler depending on how you develop it.
I think it would be really cool also to see episodes where the first two hokages are building the leaf.