I always wanted to perform mods like these on some pads I have. I bought a soldering iron to start doing it and the bloody thing melted and I gave up on learning to solder. It was probably a faulty soldering iron though so I'm going to give it another shot when I can afford to pick up a new one. I always wanted to map one of the buttons to double as Fire+Up at the same time, as well as the regular Up button, because some games like Golden Axe use Fire+Up for jump and I don't think that's been patched with WHDLoad.
Also remember you can modify a Mega Drive/Genesis pad to have full 8-button functionality, but the only game that makes use of all 8 buttons is ADoom. Flashback and Hired Guns both use a modified 3 or 6 button pad, making use of the Start and A buttons, but not the X, Y, Z and Mode buttons as ADoom does.