I know they aren't "DC Universe" characters, but the following should be in, just because the fans of DC as a whole -like me- wouldlike to see them fight to the death:

V (Vertigo is DC's graphic novel arm)

Rorschach, Silk Spectre, and Dr. Manhattan (Watchmen is the most successful graphic novel, why not pit these three against Batman, Wonder Woman, and Superman, respectively)

The Blackest Night Deputy Lanterns (Red Lantern Mera, Orange Lantern Lex Luthor, Yellow Lantern Scarecrow, Blue Lantern Flash, Indigo Lantern Atom, Violet Lantern Wonder Woman)

Black Lantern Variants (Batman, Superman, Hawkgirl)

Just a thought...