there are 32650 cells out there, very few thou, their capacity rated at 8ah max, but they are not that popular , and those do fit in D size lights
For my first gen. ROT66 with battery carrier this could be an almost direct fit
the question is the voltage sag of such a battery, it could literally blast any DD LEDs with lower forward voltage like SST20, Nichia 219C, XPG3
8 x 40mm should be good for Imalent MS36-90.2
Time to start working out my masseter (even more than I already do) for the next big things in mouth hold lights.
Problem with cells of that output power would be you can’t use springs anymore
Leaf springs are an option but they have less travel.
5 sets of double springs might handle that shock load, and amperage, but i dont think that there will be anyone building protection circuits for a 40mm cell. Honestly i am more interested in a 4x20700 lt1 max with 20ah of battery or some extra fat 4x20700 soda can lights. Most scenarios that would permit 40mm cells are going to require inbuilt charging, cars and flashlights and other. Also im genuinely curious as to if the maxwell aquisition will give many fruits of development due to ev drain chaeacteristics being the high amperage pulsing to energise motor windings
Bypassed springs should be fine though.
It would likely be cheaper. And you would only need one. Per light.
Seriously, what's the point? Used many search engines, and nothing but this.
In all honesty, if the point is to build up subtle (mental, emotional) energy around this to see if Tesla :-D gets the idea, why 40700? Most people here don't really care about a newer “40700” format, or don't know. I do know what I want, and I don't care.
Most people here would prefer and love to see improvements on current formats like 26650/26700 or 32650/32700. Concerning flashlights, a completely new cell format like 40700 means you have to build up everything around a new format, again, just for the vagary of someone else.
So then, how about spreading some other rumour like “new Power Long Battery 26650 cell, 6.5 Ah and 5C continuous discharge rate”. It could also be possible and very welcome. They seem to be quite focused on LiFePO4, though. 
$50? LoL! No way. The actual price of cells is a lot lower than what you usually see, more than probably. You can get Lishen 21700 cells for less than $3 a piece in AliExpress. At ≈3.628 times their volume, that's just a bit above $10 for an ordinary 40700, and probably up to $30 - 35 for higher capacity and power output cells.
- Not every light needs to be a hot rod. F.e. smaller and lighter LT1 with even more capacity would be a good thing.
- I don’t think this is the case. With a cell this heavy you need very thick springs anyway. With seen double springs doing 25A. Now double thickness of each wire - that can do 100A. Is it not enough?
Personally I would also prefer improvements to the existing standards rather than new ones. Though 40700 seems like a nice size, larger than 32650 enough to create its own niche. And since I’m not fond of the multi-cell lights I’d be very glad to see improvements in this area.
Though I admit, the thought of spending $$$ to replace my largest (read: the most expensive) lights is not nice. 
And I agree, the rumor is quite far fetched and I don’t look forward to seeing it implemented. If it shows up - it shows up. If it doesn’t - it doesn’t.
I think that I’d love a 40700 size cell in a small fatty flashlight with immense output or immense runtime at low modes, makes a nice lantern too.
as far as springs, sure, they’ll just have to be a bit ungraded. this is my 10A switch for 200w hotwire, switch was built by matt from ASF.
New pics seem to confirm the new format
They say that it is double the diameter of current Tesla cells (21700)
Drew this quickly with 42mm diameter :
Without play :
3x18650 : 38.8mm
4x18650 : 43.5mm
3x21700 : 45.4mm
4x21700 : 50.7mm
I have found a 40900 cell on Taobao: https://item.taobao.com/item.htm?spm=a1z10.3-c-s.w4002-21513148700.70.667c7b3dkSz6t0&id=593991411207
It is pretty inefficient, though, with only 5.5Ah. But it has only 2 milliohms internal resistance. Seems to be made for car starters.
Interesting. This is a huge news if true. Fortunately we’re likely to learn whether it is in just a week. So I’m waiting patiently.
Similar sized batteries in LifePO4 chemistry that where available over a year ago.
I mentioned them here.
Its a 5498 cell, 10x the energy of a 21700. Floats in antifreeze to warm and cool cell.
One of these cells in the blf-lt1 would give a 2 month runtime.
https://www.youtube.com./watch?time_continue=5&v=QQY2vYhysBY&feature=emb_logo