Gene,
Hate to play 20 questions, but any chance of support for 3+ li-ion cells and a high/low ring (or similar, maybe even QTC, if you're feeling adventurous) for converted Maglites? Brightness control and 3x26650 (maybe even 4x26650) in a Malklite would be pure bliss for me. I don't think anyone makes anything like that currently. Even the (scarce) 2x26650 lights (like, say, the Convoy L6, which is looking very tempting) can't swap to D's in a pinch.
An endcap that can hold a spare emitter would be pretty cool, too... mount an M61 or M91 up front, keep an M91T in the cap for when you wish you brought a different light.