I've been thinking along similar lines. Initially thought about auxiliary lights, particularly given some of the inexpensive but effective set-ups that people have described on here. But having looked into almost every possible permutation of spot/fog lamps from Hella/PIAA/Halfrauds/Ring/Trailtech/Touratech, it now seems to me that a 35W HID (from HID50) conversion of both dip and main headlamp could be the way to go. The dip to be wired to stay on all the time, to avoid a second or two of unwanted gloom while the main comes up to full illuminance and also when I switch back to dip only.

I can see two advantages: Firstly, it's a lot of light for a reasonable amount of money. Secondly, two 35W HID bulbs will provide a lot more light while requiring only 15W more than one standard 55W bulb, which shouldn't cause any charging issues.