Simply name a character that had potential to be shown in more episodes and even have seprate moments not involving naruto and sasuke.
My pick. Rock Lee.
The kid was a beast that could have been mmore developed by kishi. All his fights would be epic due to his intense hand to hand approach. No ninjjutsu or genjutsu straight to the point. Gai sensei stated he would soon be the world's greatest taijutsu specialist, but we never saw that manifest itself in shippuden. Who is your pick ?