This is my favorite modification so far, and is already available for purchase.

You can hit either pedal switch while the other is held down, and keep both switches held down at rest if you'd like; some ultra-fast bass kick speeds are now possible. Chances are, you won't be able to double-kick fast enough to outrun this setup.

Two of these units can be combined with a mini Y-jack for custom quad bass pedal setups with heel-toe action.
Each unit comes with two standard switches and mounting hardware to attach them anywhere you'd like on the base of your drum controller as in the photo (or on some other custom structure).

The switches can easily be replaced with any standard micro-switch assembly (create any style bass pedal you'd like, or you could just splice in two original RB bass pedals behind the combination circuits).

Guitar controller strum bars are typically quite noisy; they are made of plastic, they strike plastic surfaces, and use loud microswitches to detect strums.
I have a couple methods of silencing this strum bar so you don't have to drown it out with the music or play with headphones.
One involves replacing the microswitches with optical sensors.
Another involves giving the player a small section of string / cable to pluck or strum.
Yet another involves building a guitar controller into a real guitar; with repositionable fret touch sensors instead of buttons, and strum / pluck sensors.  Muting the guitar allows for real guitar players to enjoy the game without getting used to the little plastic controller.

Instead of buying an official Rock Band drum controller and finding space for it...

You could place 4 colors of tape on any surfaces you'd like, and hit them with color-sensing drumsticks instead.

A foot tap on any surface triggers a bass pedal kick; or on both feet for a double bass pedal.

Place 4 patches of tape on your desk and you can drum there.
Drum on any surface you'd like!  Drum on the couch or the bed for a minimum of drumstick noise (drumstick noise is terrible with the real Rock Band plastic controller).
You can place the tape on clothing, and drum there.  Hit your friends!  Hit yourself!  Don't hit the dog.  Your sister probably wouldn't appreciate it, either.

Instead of buying an official Rock Band drum controller and finding space for it...

Breaking twin light beams between sets of transmitters and receivers with a drum stick will trigger a drum "hit" in one direction - removing the stick will break the beams in the opposite order and will not be counted.  This also allows for velocity sensing.

Another type of system could use low power (eye-safe) infra-red lasers instead of standard LEDs, to the sense the drum sticks in the reflective mode.  This way, the hardware becomes a small inexpensive
one-piece device that can stand alone and be "air drummed".

Instead of buying an official Rock Band drum controller...

Here is a drumset that will kick your drumstick back before it ever actually strikes a drum.

Here is a video of the current prototype drum "pad", showing its ability to "kick" the stick back electromagnetically.  This "kick" will trigger as soon as the stick is detected within the pad's magnetic field.  This is demonstrated by the following video, showing another magnetic drumstick's presence triggering the kickback (much easier to demonstrate this concept through video with 2 drumsticks):
http://tinypic.com/usermedia.php?uo=W90AtR7gYK3hmmKwlT8RbA%3D%3D

A set of drum pads with this technology allows for a few novel applications.

For the novice, it can "kick" the stick at the time designated by a custom modification to the Frets on Fire game (not available in the real Rock Band game), and give the user the "feel" of the notes as they pass.

For the experienced banger, it can be valuable in situations where the audio level is not loud enough to mask the noise created when striking objects with drum sticks.

It can be operated independantly of the Playstation and computer, for those that just like to drum while watching TV.  The repulsive action is quite fun to drum with.  Optional drum sounds can be played upon hit detection when the unit is in either stand-alone mode or controller mode.
 

I would also like to make all of these controllers into standard MIDI devices.