I mean your dream flashlight that is not currently available but can be produced with the technology available today.
And not something like a "billion lumens with active cooling and a miniaturized nuclear reactor that supplies it with energy"
Bonus question: why you don't build it?
A triple or quad in mokume gain, well anything really lol, but I’ll never have one and even if I had the money I couldn’t justify spending that amount over doing something with the kids, like a holiday.
If I was super rich maybe, but that’s unlikely to happen 
My EDC is the Convoy S16 with SFT40.
My dream light would be that flashlight, but with a 10% larger head for extra throw, and an electronic tail switch with smooth ramping and a double click for strobe (similar to Anduril.)
Also, I'd like an SBT90.2 for the emitter.
Why don't I build it?
Because I don't build flashlights.
Compact headlamp in 4000K AA & 14500 compatible (with good moonlight even on 4.2v).
I won’t build it as mostly don’t have much hobby time at the moment, there was (is?) a lack of good AA + lithium drivers available in the correct diameter, plus various rumours of a suitable light on the horizon.
Edit to add: maybe something like an L6 with side clicky only and some flavour of ramping firmware.
Second edit: 10180 keychain light with single, low power aux to replace tritium fobs.
Acebeam X70GT
JKK76 2
30,000lms
Disco lights all over the body… not the least bit practical but sounds like a fun challenge… 
More realistically - a compact all in one driver with triple channel , rgb aux control , clicky and e-switch compatible AND Bluetooth programmability. One driver fits all kinda thing with easy programming via phone. (I think someone already has a Bluetooth driver don’t they…?)
This would make modding easier for me. Why haven’t I made it?? Electronics are the bane of my hobby. That and I want a dozen of them.
Hmm… I really like the 3 cell Surefire bodies. I’d like something like the Surefire EDCL2 but in 2x18500 which is also 2xAA compatible and running a 519a (of course).
Suppose it could be somewhat easily built, not sure if anybody makes a 35mm extension for the EDCL2 body, though. Plus I don’t much want to spend Surefire prices just to grossly void their durability and warranty.
RRT0SE in titanium.
I commented on this post a while back - [UNOFFICIAL CONCEPT] Emisar D7: single or dual channels, individually swappable S2+ optics, 26800 battery tube
It definitely is still on my want/wish list. It can be assembled using existing parts Frankensteined together.
I would still want it to be all throw optics with all W2’s or all W1 Amber.
well, how about a two-headed flashlight?
actually an emitter on both ends of the tube.
i have seen two or more pointing in the same direction.
also, there are “sliders” that expose a COB middle.
but this one would have one slide switch operating
both ends independently.
why don’t i build it?
then it would be my fault
if it is not what i wanted.
An ultimate zoomie that fixes all the issues with with current offerings. I would definitely prefer a zoomie to a dual channel flood/throw light but current zoomies have too many annoyances.
- 18650 battery
- XHP 70.3
- 5000k Hi CRI
- Anduril 2.0
- USB - C charging
I cant build it because our civilization has failed to come up with the technology required to make a such a zoomie. But we can send cars into space 
Two similar lights desired. A 1xAA and a 18650 twisty, each with forward clicky for momentary. Four modes; Moon, L, M, H, no blinkies. Starts wherever the twisty is set. Warm, high CRI emitter in each. S2-ish size for the 18650, UltraTac A1-ish size for the 1xAA.
slmjim
One of my first lighs was a Fenix LD20 with different mode groups based on whether the head was tight or undone. Around the same period (2011/2012?) Eagletac and others also had similar UI.
What happened to all of these light designs? Single switch multimode driver just cheaper/easier? Copyright issues? I looked up the “ultimate” rotary UI flashlight recently- HDS, saw the prices and decided I didn’t want one that much…
Wurkkos TS10 brass, but with a throwy optic and a single emitter with NW and high CRI. Oh, a deeper clip too. That would be perfect! Wurkkos, do you hear? 
Eagletac still use that UI, D3A, D25A mk ii, probably others. I have a Fenix with that UI and I both love it and hate it.
I love being able to go from low to turbo and back to low.
I love that the head let’s you bypass the irritating SOS and strobe modes.
I love that the switches are mechanical.
I hate having to change grip to get to turbo.