I'm gonna throw in my thoughts Meta Knight MU.
The neutral is played exactly like a Marth ditto, with a few key differences you can use to your advantage, and one key difference to MK's advantage. So if you want to prepare for neutral in this MU, but there are no good MKs around, grind out some Marth dittos with your local melee players, and study how to play that MU (I don't know if there are any good videos of the Marth-MK MU being played at a high level to study from), and get quite familiar with MK's instant dimensional cape.
The differences:
MK's grab range is tiny. As long as you don't make a huge mistake (whiff an fsmash), or are too predictable with your movement, or don't ever throw out hit/grab boxes, MK should never be getting a raw DD grab in neutral. This makes your shield more threatening for him, and his shield less threatening for you.
MK's much worse at attacking shields. You can actually pressure his shield with dtilts as long as you stay out of his shield grab/nair range! You can space fairs on his shield too without risk of punishment. But he can't pressure your shield, his attacks do too little damage compared to how fast they are to lock you down in your shield! Very easy to shield grab him if he misspaces!
MK's sword is transcendent, so if he does happen to swing his sword at the same time as you, he could win due to his sword hitting your exposed hurtbox. As long as you're moving and not being predictable, this should never happen.
MK has a weak tipper. Not that his tipper is sour, but your tipper is much stronger. Your tipper dtilt, for instance, does twice as much damage as his and knocks back at a slightly lower angle than his, thus he will lose more stage control after getting hit. You are rewarded better for tipper spacing!
MK must get in close for his conversions - mainly grab, non tipper dtilt, and potentially strong dash attack. You on the other hand can get conversions from your longer reaching grab and tippers. So in the inevitable wars of attrition that you will be carrying out, you will have the advantage!
These are actually big advantages in the match up, especially if you add them up. But MK has one great equalizer that can swing the match in his favor at any time in neutral. Instant dimensional cape. What do you need to know about this move? It comes out as quick as your fsmash, so you won't be reacting to it. It has similar knockback power as the knee and Roy's fsmash, so if you get hit, you're screwed. It has longer reach than your dtilt, so you can't outspace it by being a fundamentally better player. The only downside to the move is it has lots of cool down. If he whiffs, or if you shield it, you have all day to punish it. So what's the counter play?
It's fairly involved. So the way his down B works is that when the input is pressed, he starts to disappear. The longer he waits before he attacks, the greater his distance. If you dash dance in and out of your tipper dtilt range, you should be able to avoid the instant dimensional cape. If he disappears for longer, well keep in mind that he can appear in place, towards you, or away, with the choice of throwing out a slash or not. The safest option here if you see MK disappearing is to shield. He has roughly 30 frames of lag if he reappears without the sword, so you will get to punish him if you WD OoS as long as he doesn't teleport too far away from you. You can also hit MK when he disappears, so with proper and unpredictable down tilt spacing, not only do you wall this move out and interrupt him if he does try to attack you, he can't position himself to punish your endlag in time.
You can also pin down MK and not give him a chance to use the move in the first place. Remember how he's not good against your shield pressure?
Some advice regarding punish game:
When MK grabs you, he is going to go for dthrow. At the very low percents, it forces a tech chase. And a perfect MK can cover everything on reaction. So you have no choice but to mix up. Starting around ~20, you can DI up, which will allow you to act out of tumble before you hit the ground. You can jump here to try and reset to neutral with the threat of an aerial to possibly shut approaches down. If you're high enough to begin with you could actually throw out an aerial immediately. You can move the control stick to force yourself out of tumble so you can airdodge/waveland. Lots of mix ups, use them!
When you grab MK, uthrow for free utilt/uair chains from 0-60. Try to link these into the standard Marth aerial combos that end with a tipper fsmash or a spike to close a stock. After 60 your only real follow up out of uthrow will be uair and that'll work until ~100, maybe ~110. MK is a semi fast faller, so you'll have troubles juggling at those higher percents and will have to apply anti floaty punishes, which might be tricky because MK can fall quickly to throw you off, and he can put out big hitboxes.
MK has a horrible tech roll and because of his fall speed you can use fthrow/bthrow/dthrow to set up tech chases. Tech chase him towards the ledge to limit his options. tipper dsmash is perfect at covering missed tech/tech in place on reaction, tipper fsmash is perfect for hard reads or covering tech away on reaction, and up b is great for covering missed tech/tech in place on reaction if you're close, tech towards if you're in tipper dsmash range. Lots of room for creativity here! You can also use fthrow/bthrow/dthrow DI mix ups into tipper fsmash as a finisher too.
Watch for MK's dimensional cape, especially when he's comboing/tech chasing you. This is where mixing up your techs and DI is important. If he reads you, he will land this move, and you'll be toast.
Edgeguarding is tricky. If MK is recovering below the ledge, he can actually use reverse shuttle loop to place himself on the stage. So watch for that. If MK is recovering far above the ledge and doesn't require side B to recover, you'll have to apply the usual anti-air edge guard strategies. If he has to use side B to sweetspot or land on the stage, you have a variety of options and you should always be able to close the stock. First off the hitbox on MK's side b is tiny, look it up either on smashboards or in debug mode. You can outreach it and win/trade very easily. dtilt, fsmash, bair, counter, and even dair if you are slightly above are all options for beating his side B.
For stages, it quite possibly boils down to comfort and playstyle. Small stages allow you to pin down MK and pressure him with your tipper sooner, but he could also get in on you more easily. Big stages for dash dance camping wars of attrition. Platforms can benefit either character equally too.
It's a difficult MU, especially when you're not used to MUs where the opposing character is as fast and reaches as far as you (like Marth dittos). It's hard to tell who wins the MU, especially with MK's instant dimensional cape which can easily break and then shift the pace of the match. MK's punishes are quite easy to execute, so a few losses in neutral can easily lead to a closed stock at non top levels of play, which can make the MU seem quite uphill. Definitely lab your punishes for this MU. You'll definitely need it for the tech chasing, in which you can do absolutely disgusting things when you take advantage of his light weight and bad tech roll.
Edit: Updated for MK's access to instant dimensional cape (commonly abbreviated as IDC)