I agree, this sounds like a good idea since the enemy list for the battle arena ends at Itachi. It may not be top priority, but it'd be fun to see maybe Shikamaru, Asuma, Yamato, Pein, Juugo, Suigetsu, etc. However, there would be a lot of possible characters to add into the battle arena roster, and then people would end up just wanting more.
Maybe there can be a multiple enemies option, where you can go up against groups for more points, but higher chance of losing that takes 2 or 3 turns instead of 1 to participate in.