Tell you who would be great: Doctor Light, but with him using technology to manipulate light rather than superpowers. Similarly, as previously mentioned, Spellbinder.. something pointing towarRAB super-powers but explainable with technology.
Teamed up with the Riddler, either one of those would be pretty good.
One character who I think should be brought nowhere - near - this franchise is Catwoman, simply on the basis that the character has nothing to offer other than being a manipulator, thief or love/hate interest. For me, the introduction of Catwoman usually spells the end of any decent Batman-related projects. She's a bit like Black Orchid.. works great in a comic-book sense where symbolism can often explain their motives, but doesn't really have the essence of what makes a good screen character.