I too felt more could've been done with Maul. I thought it over and realized how it could've worked. Basically, after killing Qui-Gon, Maul withdraws. Then, in II and III, he becomes a focus for Obi-wan and Anakin. Obi-wan skimps on his attention to Anakin while hunting down leads to Maul. Anakin also might seek Maul as he had befriended Qui-Gon. The Jedi are all convinced Maul is the master, drawing their attention away from Palpatine, just as he hoped. At least something to that effect. Personally, I liked the Yoda instructing scene as it shows how all Jedi were, in a way, instrucyed by him. Otherwise, we'd have a plothole in Ben's line, "There you will learn from Yoda, the Jedi Master who instructed me".