If you are referring to the latest PRO PvP Survey, 96 out of 195 respondents found Stall to be too strong, so the percentage is closer to 50%, but I acknowledge your point. However, there are possibly other factors at play to explain that kind of response; I will not get into them here to avoid derailing the topic of this thread.
I hope you can see that, even if we allowed Mega Kangaskhan, it would not punish Stall as much as other archetypes that were deemed adequately strong by the playerbase. As discussed here, Mega Sableye – the premier Mega on Stall teams – halts Mega Kangaskhan in its tracks and threatens to burn it. On top of that, Regenerator Alomomola coupled with Unaware Clefable would be able to keep passing Wishes while punishing Mega Kangaskhan with two rounds of Rocky Helmet every time that it would attack Alomomola.
Realistically, the addition of a Balance breaker like Mega Kangaskhan would primarily punish... well, Balance, a playstyle that is already not in a great spot due to pressure from both ends of the spectrum. Sure, Mega Kangaskhan can be used on Balance, but what are we trying to accomplish by unbanning it? It is not a Stall breaker.
Also, keep in mind that there is no shortage of Megas to use. Players usually ask for more additions – which the PvP Council has no control over – but rarely complain about not having enough Megas to choose from.
Thank you for taking the time to read my post and reply to it as well. I value these PvP discussions and interactions with other players even if, sometimes, no common ground can be found in the end.
I will also add that the PvP Council needs a clear majority before adopting a PvP change and, in this particular case, there is a consensus that allowing Mega Kangaskhan with Power-Up Punch is not a solution to some of the metagame's problems. I shared your proposal with other PvP Coordinators to spark a discussion because, even though I personally disagree with the suggestion and find that it would not have a positive impact on the metagame, your efforts deserve to be seen and taken seriously.
I would say that the moveset used by Mega Kangaskhan vastly depends on whether you want to better its match-up against offense or defense, and so the choice of coverage options is ultimately team-reliant, but it is true that Mega Kangaskhan wants to run more moves than the ones in bold characters because it enjoys additional coverage to hit Pokémon that its STAB Body Slam or Return is ineffective against. We can note that Body Slam has a good chance of incapacitating switch-ins like Clefable or Alomomola, thus introducing an element of luck about half the time.
I want to point out a contradiction that I noticed here and elsewhere about Mega Kangaskhan forcing teams to resort to Rocky Helmet, Rough Skin, Iron Barbs, and Regenerator users. If the desired effect is to weaken Stall – even though dedicated Stall teams are far from common –, then freeing a Pokémon that would result in an uptick in the usage of the decried Pokémon, like Alomomola and Clefable, sounds counterproductive. I suspect that many conflate Stall and Balance even though they are widely different. Stall wins by attrition – by slowly wearing down the opponent with hazards, status, etc., while preventing them from making progress. That is one of the ways to beat Mega Kangaskhan.
You mention that the ability to break Focus Sash and Sturdy and hit Pokémon like Kyurem behind their Substitute would be beneficial. I would argue that Offense suffers from this. Currently, just to give an example, many Hyper Offense variants rely on the prevention of hazards with a suicide lead like Aerodactyl or Excadrill. Mega Lopunny is renowned for doing well against those traditional HO builds; Mega Kangaskhan also has access to Fake Out in conjunction with Power-Up Punch and Sucker Punch, as well as a respectable Speed tier that places it above almost all of the non-Mega HO picks. Mega Kangaskhan can turn Excadrill into setup fodder thanks to Power-Up Punch. It can even afford to stay in against Focus Sash Breloom because it always survives Mach Punch and has a good chance to OHKO it with Return. Obviously, it does not want to do that, but it can if absolutely necessary. If Mega Kangaskhan acquires a simple Power-Up Punch boost, for example against the suicide lead, it can OHKO some of the sturdiest Pokémon on HO, like bulky Manaphy and Azumarill, while surviving a few neutral priority hits in an effort to revenge-kill it. Dragonite survives most of the time if its Multiscale is intact and, while offensive Mega Scizor lives as well if we assume a non-Fire Punch set, Mega Kangaskhan can just Power-Up Punch again then KO it because Bullet Punch barely scratches its 105/100 physical bulk. Mega Metagross outspeeds Mega Kangaskhan but can fall to +2 Sucker Punch from full. For Mega Pinsir, it depends on the set; many players enjoy Adamant instead of Jolly for the extra boost, which can make it slower than Mega Kangaskhan if it is Jolly instead of Adamant. Jolly outspeeds Mimikyu and does not fear Blacephalon, Gengar, or any of the faster, Sucker Punch-weak options run on HO.
Moreover, while your comparison with Mega Mawile has merit, Mega Kangaskhan would still outspeed or threaten to +2 priority-KO most Pokémon on standard HO. Fake Out is also an invaluable safety net against Offense. There are more offensive team structures to cover, of course – Rain offense, Trick Room offense, etc.
My point is that, while it may seem like Mega Kangaskhan would help against threats like Sub Kyurem, there would be too much collateral damage. It is nowhere near as slow as some make it out to be. +2 Sucker Punch still does a number on faster Pokémon.
Your point about the stale and repetitive nature of the metagame is one I can share. As you know, PvP Coordinators are very limited in what they can currently do. Releases are not something that we oversee, although we can make suggestions to the content team like any other player can. PvP changes are a possibility, but we should not shake up the metagame without a precise purpose, because doing so recklessly can lead to chaos. It might be new and different, but it could very well result in an unhealthy, less skill-based metagame, so we need to exercise caution.
On the topic of Kyurem, which you mentioned: If you believe that it is broken, we should not free another potentially broken Pokémon to deal with it; instead, we should focus on the problem at hand – Kyurem. Obviously, the previous vote was too split, with a slight majority of PvP Coordinators voting to keep it unbanned, but the margin was slim enough for the Pokémon to remain on our radar and continue being discussed. It can still be banned if a majority is reached in the near or far future.