> i prefer it to look like a single 18650 cylindrical power bank
> with interchangeable heads for flashlight,
> … flashlight tube adjustible … to accomodate all battery sizes
What he said.
Build the important core — a ‘pill with radiator fins’ — as a carrier into which we can put any LED on one side, and a driver on the other.
Why? Because LEDs are changing very, very fast, so are drivers.
Heat radiators, not so fast. Put the money into thermal mass/fins.
Something like this: http://img.hisupplier.com/var/userImages/2014-07/17/125840565227.jpg
Modify that with the LED cutout on one side, the driver cutout on the other, and ways to attach a head and battery tube, maybe a module that’s got one or two or three switch buttons or rotary selection click rings on it.
Have a good copper/brass contact ring solidly press fit into the aluminum to make connection reliable, none of this stupid solder-to-aluminum.
Put a universal thread or a camera-type bayonet mount on each side of that core — that will accept adapters that fit various reflectors/optics/heads. Put a similar but simpler adapter on the driver side that will accept a range of battery holders.
Camera makers figured this stuff out in the 1950s — the late lamented Miranda SLR was just a fraction of a millimeter thinner than any other, and their adapters filled that space, so you could then attach many other companies’ lenses and have them at exactly the right distance from the focal plane.
Do an image search with this: camera bayonet lens adapter - Google Search
Same issue with LEDs, they have to be at exactly the right distance in the head of the flashlight. So make an adapter that will latch firmly around (not inside) the pill on the LED end and have a simple chunk of metal facing out that can be cut with whatever threads fit the desired head.
Battery side is simpler. Just do what Arc did with their flashlight — battery tubes for 1xAA, 2xAA, RCR123. And a third party even did a lovely little CR2 battery tube that fits the Arc lights.
Sure, I’m dreaming.
To improve on what Ray Bradbury said about writing science fiction:
“I write not to predict the future, but to prevent it [to make public and so prevent ideas from being patented and locked away]”