Now it's looking like there might be some real weight behind those rumors, as Firstshowing.net got a hold of a casting grid for Batman 3. A casting grid basically shows the availability and interest level of particular actors towards certain roles. In it, JGL is listed as "interested" in the role of The Riddler. That at least implies that they've spoken to him about it. This also pretty much confirms that we will be seeing Edward Nigma(the Riddler's real name) in the next movie, expected to be released in July 2012.
At this point I believe JGL can do any role he puts his mind to. No actor has been on a hotter streak of quality films than him since 2005's Brick catapulted him into the public eye again. In that span he's played pretty much every type of character you can think of, from lovestruck hipster(500 Days of Summer