That actually makes a lot of sense. I'd be very, very impressed with Mr. Nolan's foresight if it turned out to be the case. I'm not wild about Riddler as a villain, as I kind of place him somewhere near the bottom of the tier list, along with Killer Moth, Joe Chill and other essentially harmless "villains", but this could be very interesting.
I may be remembering incorrectly, but I swear there was a version of that scene where the Joker left a grenade in that guy's mouth instead of a gas canister.