If You Could Change One Current MTB Standard...

Seat post diameter, rear axle width, bar clamp diameter, bottom brackets... hard to pick one.

I would agree with your bottom bracket pick (the others I can forgive and forget)... but to up you one, how about headset cup sizes?  Ooof!

I'd kill 203/223mm rotors.  200/220 is the way.

15mm fork axles.

Bring back 20mm for single crown (Intend leading the way there as ever)

Everything else I can live with (for the most part) as there are always several options to chose from.

Not really what you are asking for, but I would like to see existing standards paired down. 

BSA BB, 148mm boost, 31.6 Seatpost, 50mm(or 52mm) chain line.

Edit to add 203/223 rotors need to go away

Yes to external cable routing. 

I like the general principle above of not tapping tiny threads into magnesium or aluminum if there’s an alternative.

However i’m racking my brain to think of any real trouble i’ve had in the last 3.5 years on my generic 2020 trail bike and coming up with nothing but some annoyance at removing and reinstalling the rear brake because of the internal routing.

3+ years without tire or tubeless frustration

3+ years without suspension trouble

3+ years without frame/bearing/etc noise

Hmm, a superfluous patch of contrasting fabric on my saddle nose frayed and S replaced it (and the new one peeled up the same way). If your job is drawing saddles i guess you’ve gotta express yourself somehow.

These are amazing times compared to just 10 years ago

203 and 233 won’t go away until a certain country accepts the metric standard 

Get frames to spec 8x30 shock hardware on every frame every time- can run standard hardware or bearings that way. More compatibility for shock swapping.

34.9 seat posts

49mm chain line (6mm offset chainrings)

Design 29” bike kinematic for 30t chain rings 

31.8 handlebar

Everything is pretty amazing now considering where we were 10-15 years ago.

I want everything externally routed. 
 

This isn’t a standard I guess, but no more rim tape. Something like the Enve strip standard on all wheels. 

external routing. as OP mentioned, Canyons original outside but coverd way was good.

SO HARD TO PICK

Headset cable routing dies forever.

Passing a law that all stems and handlebars have to be two-piece on new bikes (you can buy your stupid one-piece bar/stem combos aftermarket if you want them so bad).

All mountain bikes run a 73mm BSA BB. Downhill bike engineers, see next bulletpoint if you're worried about chainlines.

One front and rear axle standard for all mountain bikes. It could be 15x110 and 12x148 and it would be amazing. It's easy to build downhill forks with too much torsional stiffness (see: OG Fox 40), so you don't really need a 20mm front axle. Plus, the difference between 15 & 20 is extremely marginal anyway.

Infernal cable routing for rear brakes goes away forever. Zip ties for brakes.

203/223 rotors are dumb.

All rear axles get a flush-fit 6mm bolt instead of a big dumb QR or lever. Yes, you'll need to bring a multi-tool to change a flat. It's 2024. Bring a multitool on your rides.

All aluminum handlebars are 31.8. All carbon handlebars are 35mm.

TEAM ROBOT HAS SPOKEN.

The SuperBoost. If you have companies who can make 148 mm rears on DH bikes, you can make your enduro bikes with 148 too. The Millyard bike had a narrow rear end for a reason.

It's nice to have wheels that can go on any bike. Feels a bit like 1996.

Mx

I’ve got a couple:

* Long travel single crown forks - they ALL creak on me every time. Let’s go dual crown.

* Forced 55mm chainlines on boost/148 frames. SRAM trying to push the industry to 157? In reality I know it’s better for e-bikes that live in the small cogs, but not ideal for earning your vert in the 52t. 

* 35mm handlebar clamps.

Dh bikes and forks using the same boost sizing as trail bikes.  Quite a few bikes have gone this route it would be nice if everyone including forks was on the same standard.

My luke warm take, one metric Allan key or Torx bit size for all handlebar controls, and not something too small.  Three, sometimes four different sizes just to replace a handlebar is annoying AF.  
 

Also, all rear shocks have a single mounting hardware size (likely not feasible) and Fox type (get rid of DU bushings forever).  

Uhm, except you have it bass-ackwards: Imperial nation SRAM uses 200mm and Metric nation Shimano uses 203mm (which I have never once seen labeled/marketed as "8-inch" rotors).  Afraid you've got the wrong conspiracy: it's not nation states, it's capitalism: Shimano wants to lock consumers into their system and prevent cross-compatibility w/ SRAM et. al. wherever possible.  Need more evidence? Spend a little time reading up on cable-pull ratios w/ shifters & derailleurs. Shimano started playing those games to stick it to Campy at the dawn of indexed shifting.

Superboost 

Cable headset tourism

Pressfit

I wasn’t talking Sram vs Shimano. Or any “conspiracy” you want to try and throw at me.

Doesn’t Sram have a pretty big operation in Germany though?! Hence the propensity for metricness

Separate bar and stem combos – they should all be one piece. 

But seriously, ISCG-05. All three tabs on every bike and frame designers following a uniform standard for clocking and distance from centerline.

A single direct-mount chainring standard and a single BCD standard for spiders (104).

The single standard I'd sentence to death as king of the world: headset cable routing.  We get to the point where modern suspension is so good that headsets essentially last the life of a frame, so the dirt-roadie fraction at Scott decides to run all cables on all MTB models through a dirt-funnel into the headset because Nino will save 1 watt on aero gains on his XC bike.  The sheer structural stupidity of it makes me scream even before the home mechanic starts cursing at the extra work of dragging cockpit disassembly/re-alignment into simple hose & housing installs/replacement.  Next on the chopping block, for likewise causing senseless aggravation: mixed Torx & Allen bolts in single component (looking at you SRAM, I need 6+ different tools to adjust the SRAM components on my bars alone). Then seat post sizes: 30.9 & 34.9 die, 31.6 remains. Then rear hub width: 148mm lives, Pivot & the Stupid-Boost+ conspirators are banished.  Bottom brackets next: as with 148 boost, BSA is already de-facto choice of sensible majority.  Press-fit dies, and while we are at it, DUB's head rolls and 30mm becomes crank-spindle diameter of the land so royal subjects can mix & match best BB w/ best cranks for their application.  The king is now tired, and will retire, even though there is still much God-ordained work to be done.

Superboost for sure, good wheels are expensive and there are many bikes I won't even consider because my nice wheels won't transfer easily. 

SuperBoost can GTFO!  If DH bikes can be made to work just fine with 148, why does Superboost exist on trail bikes?  I often swap wheels between bikes, I have one set of "enduro" wheels that I've used on at least 4 bikes.  Currently, all my bikes, other than my fat bike and dirt jumper have 148 spacing and it's come in handy.  Last summer, I had a rock go through a rear wheel on my trail bike 3 days before leaving for a 10-day riding trip in Iceland, I broke 3 spokes and knew I wouldn't get the wheel fixed in time.  I borrowed a set of wheels from another bike and didn't miss a beat.  Because of this, I haven't even considered buying a new frame that is Superboost, I was sad when Evil switched to SB, I had 3 Evil bikes in a row but couldn't bring myself to make the jump to SB.  

Glad to hear someone else feels the same way about buying a new bike and Superboost.  I've wondered how many lost sales certain brands have because they are Superboost.  Is anyone out there buying a certain bike because it's Superboost? 

Echo some of the ones people have already listed. Superboost, cable tourism, bb standards and rotor sizing.  

I'd say torque caps are another one that I want to see either adopted across the board or eliminated.  I have an equal spread of rockshox and fox in my fleet of bikes... I recognize that torque caps are superior, I believe that I can even tell a difference in stiffness of torque caps vs non on 35mm stanchions and lighter forks.  It's annoying to have a different standard for hub end caps... maybe I'm just jaded because dt's torque cap end caps suck.  

Overall bikes are pretty amazing right now and I have to work pretty hard to nitpick the products coming out and the industry as a whole.  I do think that generally, some of these standard questions start to highlight how complicated the machines are and how educated you need to be as a consumer to not screw up.  Very easy for jerry's to not understand that there is a 200mm rotor and a 203mm rotor and they need the proper adapter.  I can't blame them for the confusion because you really need to be detail oriented and have a minimum of a 20% engineering mindset to fully understand the f1 car of a bicycle you're riding on a weekly basis.  

As long as we're getting rid of 203 / 223 rotors, let's do away with centerlock too.  A solution in search of a problem, that mostly just ends up creating more problems.

TimBud wrote: Doesn’t Sram have a pretty big operation in Germany though?! Hence the propensity for metricness

SRAM bought German Sachs for derailleur manufacturing capabilities, has nothing to do with their brake designs & standards that came from Avid, founded in California over decade prior to SRAM acquiring them. 
Just so we're clear, the reference to conspiracies was facetious (as marked by humor of intro, but keyboard sarcasm is eternally damned to fail somewhere in the reading).  The point is, US core cycling consumers in this millennium *do not purchase or think of components in imperial* any more than their German or other global counterparts.  Wheel & tire sizes keep imperial nomenclature alive worldwide, even though our "29er" tires are actually rolling on 700c road standard rims. But even the propensity to market frames in inch sizing died over 30yrs ago, as manuf. moved to T-shirt sizing to fit consumers in S, M, L, or XL.
TLDR: outside of wheels/tires there are no remaining market forces in US that drive marketing of imperial units in bicycle standards.  Brands chosing an "imperial remnant" norm is purely a matter of divide-and-conquer-market-share strategy.

Chain rings! How many options do I need for this stupid metal thing. I have at least 5 chain ring/bash guards that are gathering dust because I ordered the wrong size. For a wear item, it’s too many options. 

"All aluminum handlebars are 31.8. All carbon handlebars are 35mm."

Why? 

I forgot about Centerlock!  I hate that too, mostly because there's no way to tighten them trailside if they were to get loose.  Granted, I've never had one come loose but I hate the idea that they require a tool too large to fit on a multi-tool.  

Nooo! "Screw" torx. Seriously, I have been guilty of using torx screws in stuff that I have designed and I can tell from experience: there is absolutely no advantage in real life. Torx screws do not last longer and these rounded-flat-head bolts used for rotors are the worst! We could talk about multi-tooth, these are nice, but really we're trying to solve the problem that does not exist. Buy quality tools (hazet, swisstools, stahlwille etc.) and you'll be fine.