So I will say straight up, I am a noob at BW OU. Despite having interest in the tier for a few years now and it being my second played gen, I do not have the skills to win in this game at a high level. But I'd still like to get my thoughts out there.
Thudurus-Therian and Cloyster Ban
I'm going to lump these two together because to me they are similar in what they do broadly and how I think they should be dealt with. In my eyes, they are both gimmicky ass mons that do not contribute much to the tier and cause headaches in the teambuilder. Thundy-t at least helps rain out, but I don't think that's enough to keep it in the tier because yes, I do think they should both be banned. Rain was dominant for so long and it taking a back seat in the meta isn't really a bad thing in my eyes and can allow for more creative teams to rise. So, Thundy-t helping a playstyle that used to be dominant by forcing so much RNG with thunder paras and such, isn't really that good. Cloyster is just chesse, straight up.
Latios Ban
This is one that I am a bit iffy on. Latios is a core component of the metagame, and removing it would definetely impact the tier. Keldeo would straight up need to be banned in my eyes. I don't think shaking up a old gen tier that much is a good thing. However, I do believe that Latias would fill some of the void it left. Of course, latias is MUCH less powerful than Latios, and as such latios' revenge killing capabilities would not be able to be replicated. However, latias has much better defenses and as such would be better in the defensive department. It would not replicate Latios 1-1 and most likely some things would need to be banned. But I think seeing how latias could replicate latios' qualities could be good for seeing how much an impact removing latios is. Latios in itself is an offensive beast and difficult to deal with, so there is merit to seeing it out.
Sand Force Excadrill Ban
I do not think that excadrill should be banned. It is the most splashable spinner in the tier by far as while donphan and tentacruel are great. they fit really only on sun and rain respectively. Excadrill can fit on any playstyle and that in itself is. I will say that I don't think that a spikes ban would be a good idea, but excadrill is a good thing for stealth rock removal, and if we remove both then there would not be a lot of progress making. However, I can concede that sand force excadrill is somewhat of a problem. I think in this scenario, the bending of the rules should be done a bit. Ban sand force, I know that this has been dismissed but in the past these sorts of decisions have occured and I think that in a metagame as old as BW, these are alright. If this was current gen, I would oppose this but there is a status quo to uphold. Excadrill is a key component of the tier, and as such it should not be banned, only sand force if that's allowed.
Lugia Unban
Despite my vehement opposment to unbanning Lugia in SV, this isn't the worst idea in the world. Lugia has a lot of issues, mainly its stealth rocks weakness and sand weakness, while also being weak to both of tyranitars stab moves. However, I do not believe that we should introduce something that could potentially be broken in a tier where there are already potential brokens running around. Wait until their are more bans, then discuss this.
Deoxys Defense Unban
This straight up is a dumb idea. I won't have much to say about this but Deo-D will not be using defensive sets, it will revert to the hyper offensive spikes lead set and it will terrorize the tier once again. Do we not remember how it warped the tier so much before? Don't do this, it will only lead to pain.
Unban Gems + Ban Volcarona
I do not like this idea. Even if we hypothetically ban cloyster as well, it is not a good idea. Gems straight up are broken and reintroducing them would be a major step back in tier development in my eyes. They are better z-moves, which were already contentious, however you can use multiple of them per game with no drawback. Volc I think is only an alright mon, and although I do wish it were banned, doing it on the basis on unbanning gems is not a worthy compromise. Do not do this, this will not bode well for the tier.
Ban spikes
Spikes I am against banning (side note, this topic I am the least knowledgable about in BW, so take this with a grain of salt). As a whole, I think spikes are a fundemental part of pokemon and banning them would not be a good idea. While it does reduce defensive mons ability to wall things and it increases the usage of psyspam teams, I don't truly think there ban would solve as much as people are claiming. While the above statements are true, I don't think that spikes are fully evil. They also of course limit offensive play, but that's a cheap arguement to keep them because everybody and their grandma knows that, but I also think that psyspam teams with reun, zam and latios would not lower in viability that much. Yes, they would decrease in viability, but two of the three ignoring stealth rocks and sand chip is amazing in this tier. Ultimately, this topic is the one I am least familiar with, but I don't think spikes should be banned.
I am a noob in BW OU, so take all my opinions with a grain on salt. But if you want a tldr list:
Yes: Thundy-T and Cloyster Ban
Maybe: Latios Ban, Excadrill Ban
No: Lugia Unban, Deoxys-Defense Unban, Gem Unban+Volcarona Ban, Spikes Ban