Rock Band 2 drum set. Sandwiched between the orange kick pedal and multi-colored cymbal ports (see above), the extra jack was initially explained away as an expansion port for a second kick pedal, as corroborated by a review guide sent to OXM UK: "It was stated in prior interviews/demos that this input was a jack for the second kick pedal," begins the explanation, before ending with the kicker, "which is not the case." Dun-dun-dunnnh.
The guide continues, teasing that Harmonix "has not revealed the functionality for this input," which will "be announced at a later date, along with further details for double-kick functionality." So ... the port is for a second kick pedal, after all? Along with a ... what? Perhaps Harmonix is vying to make good on one of a couple shelved Guitar Hero add-ons? You tell us.