re: What MUs Fox might lose
I can only really speak about the MUs that I play in my region (NY/NJ) because that's mostly the only experience I have.
Jaden, Malachi, and I all came to an agreement that Fox loses to Peach 55:45. The introduction of AGT makes it so you can't camp peach as effectively as you used to be able to, plus the nerf in laser damage makes laser camping less viable in general. Peach also has way better edgeguarding because she can float cancel from the ledge to cover more options than she used to be able to. Fox having less kill potential isn't that big of a deal, but Fox needs to outplay Peach in neutral one or two more times than he used to, which is kind of an issue. With laser camping nerfed, the neutral isn't as heavily in Fox's favor as it used to be, but circle camping with top platform is still just as effective as it was in Melee, so he still slightly wins neutral in my opinion. The punish game and edgeguard game as heavily skewed in Peachs favor, so slightly winning neutral isn't enough of a reason for Fox to go even or win the MU, hence why we think it's 55:45 for Peach.
Fox could potentially lose to DDD, but I don't think the MU has been developed enough for me to have a solid opinion on it. As of right now I personally think it's 50:50, from what I've seen, Fox wins the neutral really hard and has a decent punish game because you can juggle him at low percents and kill with drillshine usmash or uthrow uair. Maybe the neutral isn't being played properly on the D3s end in my region, I'd probably have to play ripple a bit to get a better understanding of how the neutral is played and what options each character has out of neutral. I can't look at Ripples wins over people like Westballz, Lucky, etc. and use that as justification that D3 wins the MU because there's a lot of counterplay that those players aren't using against D3. There's no top level Fox players that are well versed in the D3 MU, so it's largely an underdeveloped MU and I don't want to concretely put a MU spread on it, because it's probably wrong. (Lucky's a good player and knows PM matchups, but he really only knows the MUs that people play in Socal)
The only good ROB I've played against is Bstuk in a MM. I also think this MU is extremely underexplored from the Foxs end and putting a number on the MU spread would just be a guess. I think that BaconPancakes could tell you more about the MU than I could since he lives in the same region as Bstuk, but any high level Fox players that I've seen play the MU just have no idea what to do (i.e. Mojo vs. Drinkingfood) because PM isn't their main game and they don't play the Rob MU ever. I think only after playing DF would I have a good idea of how the MU is played, but odds are that'll never happen. rip.
MK is a character I think could beat Fox 55:45. These two characters just beat each other up. The neutral is evenish in my opinion, any option Fox has out of neutral, MK has an answer to, and vice versa. The punish game & edgeguarding game is slightly in MKs favor. MK has very flowcharty edgeguarding so it's easy to guarantee Fox not making it back to the stage once he's below the ledge, and MK can cover side-b very effectively considering he has a 3 frames ftilt and a dtilt. MK theoretically has 0-death tech chases on reaction against Fox, and even without that, his punish game is still very healthy. There's probably a lot of counterplay I could do that I haven't thought of yet so the MU spread is prone to change.
MUs I think are even or roughly even in the current meta: Marth, maybe Ike, Falco, Pikachu, Snake(?).
MUs I think Fox wins even though a lot of people think it's even: Sonic, DK, Diddy, Lucario,
The issue with talking about Fox MUs is that any top level play is just the opponent knowing the MU while Fox doesn't know the MU, so people think Fox loses the MU meanwhile it's just the Fox that doesn't know the MU or proper counterplay. So talking about Fox MUs is tricky. I don't even think my opinions on Fox MUs has much validity to it since I don't consider myself a high level Fox player either.