1 x 18650 with the option of an extender tube. Thick body (25mm) and a head with with smooth curves, maybe 30mm to keep it fairly slim but very solid. Neutral with a light OP reflector. Both a side switch and tailcap switch for simple on/off in various grips, allowing momentary on. The light should have several modes, which would be chosen by multiple head/tailcap twist, or even via usb (charging port would be outstanding). One mode for high/low (600lm/30lm), another for strobe/low, and another for moonlight/medium (maybe 0.2 and 200). A simple 1/8th turn twist of the head or tail would change between the 2 levels of each mode.
With these features, I would use it at work for searching houses, yards and cars, and it would make a fine light to use with a pistol. Then I would change modes and it would be the perfect light for camping, outdoors, exploring and just around the house. This sort of light would appeal to emergency services, military, campers and anyone who wants a tough house light.
If you build it, I will buy it.....