First off, I think its great that Ubers is taking steps to solve the problem, and hopefully this is our path towards a better and tier in general.  Now my personal thoughts on the matter haven't really changed since around the beginning of UPL, which is that USM as of now has a lot of problems as a tier because teambuilding is far too restrictive, and some things like threat stacking make teams always have some sort of matchup problem.  I think a large part of this originates in the fact that there are too many large threats in the meta to truly cover in one team, and you often feel that you are either spread thin (maybe like 2 fairy resists or smthg) or have glaring weaknesses in certain areas.  I do have the belief, like many people above that banning things like Primal Groudon and Duskmane directly isn't the solution however, and that is mostly because the effects they would have the meta aren't necessarily beneficial (banning pdon for example would also probably lead us to ban pogre), and also because I believe like Nayrz that we should strive to do a minimalist ban list.  So to move on to where I actually present what I believe the problems in the metagame are Solgalium Z, Gothitelle/Gothorita and Geomancy.  I am proposing to suspect any one of these, not all at once just for clarification, and each provide different routes to allow more diversity and balance to come back to the tier. 



I'll go through each one by one and why I believe that they could significantly help the tier if they are banned.  The first is Solgalium Z, which may seem at first like a weird pick, but hear me out.  I believe that duskmane's main problems right now lie in its Double Dance set, which can sweep many teams easily and it can notably break through the majority of its resists pretty easily.  And in large part this is because of Solgalium Z, as without it, pokemon like defensive yveltal can check it (the 10% drop from solgalium to steelium is pretty big) and other defensive checks like primal kyogre, hooh and the like aren't pressured as hard by the duskmane who will suicide zmove into them.  While it may seem minimal, I think it will make a big difference.  It also is notable because without the zmove, defensive sets can now be trapped by mgar giving it a more defined weakness.  A big reason why I want to reduce the impact of duskmane is the current strain on teambuilding it provides.  Much like xerneas, you must bring multiple resists often times and it can be disgustingly good even when prepped for (unlike xern its much harder to play around and can run many more viable sets that you have to play around in different ways). 



The next one is Goth, which Nayrz already mostly went into.  The reason these mons are so disgusting is because unlike mgar, the other stag user, you dont have the 1 turn buffer which can be really huge as you can often force the opponent to trade a mon to be able to mega or at least put pressure on him.  Goth can often eliminate key components to a defensive core and this has been really troublesome as of late and has caused stuff like DD Zygarde to become a large threat because many of DD zygarde's checks can be trapped by goth pretty easily.  On top of this, it feels very much uncompetitive when your opponent has to only make 1 read out of like potentially 20+ turns and they get rewarded for making that one play, when you often don't get rewarded.  Going back to a teambuilding perspective, Goth is one of the biggest offenders because it limits the ability to run a lot of defensive cores because they are in fact weak to Goth + some threat or similar.  While, Goth by itself can generally be agreed to not be an amazing mon, its the threat of Goth that is really scary and this can force you into bringing suboptimal stuff just to deal with it. 



The last one is Geomancy, which seems kinda childish or nooby to suggest, but Geomancy for a long time has been a huge issue in the Ubers metagame.  Prior to this, because it was one of the few threats you had to deal with, it could be dealt with as devoting a slot and a half to Xerneas was fine, but now that you have to use those slots to also cover new major threats like Marshadow, DD Zygarde, duskmane and mixogre, it makes it much harder to safely feel like you aren't Xerneas weak.  I think that geo isn't even close to Xerneas' best set in the current meta and removing it will still see Xerneas be a top tier pokemon.  I also think that Geo isn't really a problem by itself as we have seen in the past.  It is moreso the combination of all of these large threats coming together that you have to prep for.  However, I will say that Geomancy is probably the biggest target or one of them (duskmane being the other major one), and that eliminating geo will reduce the strain on teambuilding.  Think about it this way, right now you have to cover all the threats of Duskmane, Zygarde, Xern (both sets), Yveltal, Pdon (2/3 major threats), cm arc forms, ekiller, marshadow, Pogre (2/3 sets) among others, and some of those mons listed near the beginning require a specific slot or multiple slots dedicated to stopping them.  With Geo gone, its a huge strain off the building process. 



TL;DR Problem in Ubers rn is that teambuilding is far too restrictive, and too many threats are in the current metagame to really cover.  2 ways about it, either target the actual threats (Xern, Duskmane) and ban specific "problem" threats or ones that are hard to cover, OR ban goth and allow a lot of the defensive teams and mons a lot more breathing room when forced to check many of these mons.