Thanks for your responses, everyone. It's been very informative.
The reason I asked about a "from the ground-up" flashlight, was because I am interested in getting a "serious emergency" kind of flashlight for my parents... that could run for many hours at high intensity, but would also have a low intensity setting that could run for much longer. Looking at the "low" setting of the L2D-CE, and its 60-hour runtime with NiMH, I wondered if a 3-4D cell light with the same type of multiple modes and regulation would have 300+ hours of continuous light on one set of batteries.
The MAGs are nice, but they will only run ~20 hours, because there is no low setting.
IMO, a lower powered light (>10lm – possibly around 30-40) with alkalines is what I'd be looking for. I can see the advantages of having more then one brightness level and for rechargables batteries, but when the power goes out and you don't know when it'll come on again, rechargables have already been losing their charge while stashed in the light in a drawer (unless right off the charger) for possibly, months doesn't help and neither does actually using the light because that's going to drain an already lower charge. To me an emergency light is one that sits and waits to be used; nothing more. Using alkalines, it'll take years to get at the low voltage levels rechargables can get at in a few months.
In my last post, I recommended two of the brighter drop-in bulb replacements, let me recommend one other; everled. Not nearly as bright as the first two I talked about, but will last, I'm guessing, around 24 hours in a 3D light before you can see a really noticeable drop in output. While that's not your wanted 300+ hour runtime or 60 for the L2D-CE, but even after you see that drop, it will produce usable light much longer and most likely at the same level of the lowest level of the L2D-CE. Paired with a 3 or 4D maglite with alkalines and you're looking at a very decent emergency light IMO. Buy an extra pack of batteries or two (come in packs of 4 and 8 here so just make sure you won't have extras) of D alkalines and I think you'll have plenty of runtime and light to boot. If I remember correctly, putting everled in a 3D light will make it produce as bright a light as it's ever going to and adding extra cells (can run with up two 6 cells) will only increase runtime.
Another thing I think worthy of mentioning is the runtime of both the modded maglite and L2D-CE. The maglite with the drop-in bulb replacement I mentioned above will not produce as bright a light as the L2D-CE can, but will last much longer while giving out more usable light than the L2D-CE. Basicaly, what I'm saying is the drop-in modded maglite will still be producing more light that can easily be used while the L2D-CE will be only able to maintain its low setting.
I'd like to hear others opinions on this.