It's been a popular topic in UU discord for a while but there's definitely a fair bit of desire to test Muk alongside Aerodactyl in this for GSC, and that they should both be being considered if aero is in general for tiering changes. As an option its had a fair amount of play testing atm to already start proving itself a valid option to test here and its by far the best thing we could consider adding to the tier in its current state, even if you dont do anything with aerodactyl.
Muk helps alleviate a fair bit of the problems people have had with linear building and passive play with the extra boom and offense it provides, its not objectively broken after a fair bit of testing has taken place on it lately and has potential to actually fit into the meta pretty nicely, there's some nice counterplay around between all of like omastar, (aero if we test it), magneton, haunter etc and while it has moves to get around answers its not necesarily able to do this all at onec and we have ways to punish the boom.
The main concern with Muk on the face of it that I initially had was that it has the potential to break mime and electabuzz, since teams just would not necessarily be qualified to handle these after a muk + haunter combinatio could force too many trades. While i do share the same opinion this is a potent offensive structure, there is a lot of skill expression going on both from the player playing such a structure and the players who have room to play around all of this etc, its not brainless in the slightest like BP used to be and actually would make the tier more interesting to watch.
Muk has another benefit worth noting when we drop it into the tier, its raw bulk + ability to force trades gives players more variety in options to handle stuff like scyther which is fantastic,again this diversifies muk a bit too and forces it to not necessarily want to boom earlier because the thing actually thrives better in mid-late game when its got more opportunities to start doing curse stuff and to do these valuable trades.
Two questions sorta lie here then,
-> Why is this a worthwhile addition alongside Aerodactyl?
-> Why is Aerodactyl worth testing if we drop Muk?
To answer the first one, Aerodactyl certainly has concerns alongside the drawing the tier more in the passive direction that people have been complaining about. It actually makes harder progress for a lot of the tiers neater offensive options like Dodrio, Victreebel, and Electabuzz in practice that games have shown, and forces more undesirable awkward granbull sets and the like too. While you could view it as helpful for worsening scyther and limiting other cool offensive options, the reality is its limiting the ability to make progress as well. Muk is perfect to actually balance this a bit, you get the defensive benefits that aero brings to the tier but you gain another great option to punish it and actually synergise with the offensive breakers so that the tier isnt so stifled offensively.
For the second one, Aero actually helps limit some of the concerns muk might have through the addition of another normal resist thats not quad ground weak or physically frail like magneton / haunter, it has more likelihood of forcing booms too. While all of this is true the thing isnt some perfet answer still, +1 explosion will just take it out of existence and thunder can still do things, but its putting more limiters on muk while being a more unconditional answer to things like scyther etc while muk acts more as the offensive back up option that will always trade with it if kept reasonably healthy, so we get the benefits of aero without its negatives on teambuilding and game slowing with some help on making muk seem more reasonable for those who kinda doubt at first. Important to note here that people may still potentially consider things like scyther a bit too much with muk alone (i honestly dont think so personally) so aero actually still has that value when you are doing this.
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The overall gist here of why muk should absolutely be considered here is that it diversifies offensive options and adds a neat new breaker and enabler that helps check offensive threats, helping push the tier away from boring psychic/water 1v1s that people hate watching and adding more skill expression and more ways to punish shit like stall. Its also helping to ease the valid concerns good players have had with aero on its own and both drops make each other more palateable to consider.