Hard to know where to start really. If we’re imagining impeccable manufacturing then you could hugely outdo the competition in any number of areas. A robust miniaturised cooling system? You can offer twice the sustained light output of anyone else.
EIGHT AA ? That’s looks huge, also does USB charging work for NiMH?
For 4S NiMh you need a Buck/boost charging circuitry for that chemistry as you need more than 5V
What would YOU come up with?
A new model of the BLF X5 or X6 SS with CU heatsink
I would focus on a very clear goal and optimize for a use case, not try to maximize specs, and explicitly resist chasing non-priorities.
For an actual example of this in progress, see: [Group Buy Interest Poll] Tiny EDC keychain light done right - BLF/Sofirn-style! - #4 by phouton
Hire some engineers that know more than me. 
I like small compact pocket lights, I designed a couple if anyone wants to make them :)
CRX Copper, Ti & CF Pocket Rocket:
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CRX Copper & Ti Pocket zoomie:
I thought a really well made zoomie with nice materials might be popular.
A GT mini with a 21700 battery, SST-40 de domed
I believe USB-C charging with a power bank function is the future. Hardly a bold statement. Acebeam came close with the EC 35 II but it fell short in a few areas. A similar flashlight with 21700 capability, a TIR lens coupled with a high CRI emitter and a better deep carry clip would be the ticket for me.
The CRX zoomie looks awesome to me!
The once ubiquitous cheap 6V floating torch/flashlight would probably benefit from a “BLF” design from scratch. I can envision multiple 18650s, USB recharging, a single efficient NW emitter. Limitations would be an all plastic case (internal heatsinking?) and weight no greater than the 6V version.
It might be hard to beat the price of the cheapest offerings but I’m sure there’d be a market for a more premium version.
I want wrist flashlighs. Just like wrist watches. Detachable, head bendable or angle adjustable and has a small slot for changable battery(probably 10180 size).
I’d create an exact replica of the M18 Maverick by Olight. Best small-form 18650 working-man’s light built to this day IMO. Perfect UI, almost indestructible, runs forever on a single battery or so it seems. What really sticks in my gut is that this flashlight was in the “discontinued” closeout column at Illumn.com for weeks and I didn’t snag a handful of them. Already had one, didn’t need another,right? Olight will be coming out with something even better after this one sells out, right?
Well…….no, that was not right. I dropped the ball there. Olight has built and continues to build some fine flashlights but they have not filled the gap that was left when they discontinued the Maverick. Nobody else has either from what I’ve seen.
Well since we are talking fantasy I would create a flashlight that was thermally stable at all its power settings using 6 21700 cells with unreal run times and had a head diameter of only 52mm’s. It would have to have an led display giving power output, battery level, time and date as well as being able to charge other devices. It must have a tail switch with a thumb paddle for instant turbo or moonlight. Power switch must glow when on or off. A big magnet around the switch and along the thick body for working on vehicles or whatnot. Smaller in diameter then a can light and 20mm longer. Something like that off the top of my crazy head. Or how about a round flashlight you could attach to your chest with unreal runtimes. 
On a more realistic idea. Why not take what Acebeam did with the W30 a bit further with a twist. A zoomie with 1500 lumens and no square looking beam. 3000 meters and multiple lumen settings. Make it the best flooder with unreal throw capability. Is designing a lens really that hard with todays technology. It would still have to have a large power bank for long run times. I would like to have the acebeam W30 but at only 500 lumens and 329 dollars it’s way more then I am willing to pay. I have the L30 and the X45 that I payed full price for and they are great torches. Most flashlights seem to all be the same without stepping outside the box. a hardcore zoomable flashlight would be cool.
I’m with jon_slider something similar to a Jetbeam TCR-1 if I wanted a sought after light.
Given you asked for something that would appeal to a large number of the community and be at a low price point.
Just my opinion but the Drop AAA tool seems to have been a huge success. Offer a different design maybe with a higher lumen output and options for good binned leds.
It could also be made to be compatiable with a 10440. A custom UI might even be another good option but low, med, and high seems to sell good.
I’m much more likely to spend $25 bucks on a good light and end up getting more than one, sometimes several, than I’ am spending $50 or $75 bucks on just one light.
I would design the “perfect edc”. Something like what CRX designed. Basically an Olight S1 Mini scaled up to take 18350 and Andruil.
Single LED with TIR. Emitter options would be Osram WF (maybe with a reflector) for the cool white thrower, Samsung LH351D “Dog Farts” for the neutral option, and Nichia 219b for the fans of rosy tints.
I would also make it very mod- friendly so that it could easily be updated when better components inevitably are released.
I’d design a flashlight optimized for normal people.
–21700 battery
-USB charging
-USB Power Bank
-Compact straight ‘tube’ format
Most Important:
-Simple High / Low interface based on Head tight / Head loose. Head Tight = ~1000lm x 2hrs; Head Loose = ~100lm x 20hrs.
Have runtimes and setting clearly marked on the bezel, so that someone with no instructions can figure it out.
High x 2hrs <———> Low x 20hrs
That would be the light I’d buy multiple of to give to friends and family.
Instead of a charging port a flashing port. Or at least micro USB / USB C port on the driver.
That might encourage numpties like me to get into flashing