5. Mister Mxyzptlk or Bat-Mite
Both Mister Mxyzptlk and Bat-Mite are Imps from the Fifth Dimension - a dimension which is not bound by the physical laws we are familiar with, enabling them to seemingly do things that we perceive as magical (such as warping reality itself) on a universal scale. Mxyzptlk is a fairly frequent, mischievous foe of Superman's - who can only be defeated by having him say his own name backwards - while Bat-Mite idolises Batman. Too similar in concept to separate into different entries on this list, it would be very funny to see either Mister Mxyzptlk or Bat-Mite appear in a Superman or Batman movie.
How could they cameo humorously? In so many ways! I'd personally love to see either of them appear
during the credits, breaking the fourth wall and speaking to the audience, merely as a light-hearted Easter egg in a Superman, Batman or even a Justice League movie. But it would also be funny if Mxyzptlk was to appear during the movie, only to be casually and immediately tricked in to saying his name backwards by Superman and never being heard from again. It would also be cool to see Bat-Mite lurking stealthily in the background of a Batman action sequence, so that only viewers with the keenest of eyes see him on their first viewing - since Bat-Mite loves to watch Batman in action in the comics, this wouldn't be too far-fetched (to have him more readily involved in a Batman plot would totally ruin the mood of a Batman live action movie). The fact that their existence in a live action DC universe could indicate the real possibility of a future Emperor Joker movie would make fans pee their pants with excitement.