Well, if anyone wants my total fantasy light:
Small - about 2/3 the length of Arc AAA with same width.
Variable focus - from pure flood to pinpoint laser; however this could decrease the ease-of-use - when coupled with the variable brightness, it would need two places of rotation, though I suppose one could be at the front (focus) and the other at the back (brightness);
Infinitely variable brightness - the more you twist the head the brighter it gets (with a logarithmic scale), with max brightness at least 1000 lumens, low coming on at about 0.5 lumens - no more than 2 full turns from off to max.
Long runtime - at least 24 hours on max brightness.
Rechargeable - a single rechargeable cell, rechargeable from within the light; the light should come with spare cells and an external charger; charging should be instant (or as fast as can be without tripping my circuit breaker) (note, a solar charger would not be as useful as extra cells here, because even if it was perfectly efficient, it would take days to charge the cell unless you had a huge solar cell).
Cool running - should produce no detectible heat (by a human).
Durable body material - Diamond is the preferred material here, though tungsten or any very hard material would work - the weight of the material is not going to be much at this size - lightweight is good, but a little bit of heft is fine too.
Full body knurling - a few unknurled rings might be okay; i prefer pretty grippy knurling, like Firefly 2.
Lots of colors - idealy I'd have one of every color I want, but black is a must.
Tritium locator - should have tritium (or equivalent eternally glowing substance) ring at tail or between rotating portions; not too bright, but enough to find in the dark.
Cheap - free is even better.
Upgradeable - user-replaceable modules are best (such as changing beam color), manufacturer upgrades are fine too.
Tailstand capable - for candle mode if I ever need it (though I never seem to).
Keychain attachment - a small split-ring is fine, and it should retain its ability to tailstand while keys are attached.
Waterproof - capable of going to the bottom of the ocean.
Headlamp capable - should come with easy to use headlamp attachment.
Readily available - I should be able to obtain as many as I want as soon and as often as I want.
There you go. This exact light will come out the day after I die of old age.