Here are a list of mons I believe as noteworthy suspects:
1. Jirachi - With the inclusion of Tapus and Salamence it has gotten so much better, and is definitely a cut above the other redirectors
2. Azumarill - too easy and too powerful of a win con. Doesn't need Jirachi to set up and take out 2+ mons. Azumarill can be totally useless in the hands of a worse player, but when both players are equally strong, I believe the person with Azumarill has an advantage. Azumarill also make the game more match-up based imo, punishing players who utilize slower-paced, bulkier teams compared to those with a more offensive, faster-paced build.
3. Hoopa-U / hyperspace moves - I think hyperspace moves are broken by principle. A move with solid base power with the ability to remove protection is absurd if you think about it. Hoopa-U with speed control (whether it be Trick Room or Tailwind) can be an uncontrollable wrecking ball alongside another offensive partner. Whether we suspect the hyperspace moves or Hoopa-U itself is up for discussion
4. Tapu Lele - If Jirachi ends up going, then I can definitely see Tapu Lele becoming problematic. Lele + Deoxys-A would dominate even more with one of its major counters removed. Psychic + Fire is such a lethal offensive combination that can be hard to stop without a select few mons on your teams. Also forces teams to run other Tapus to counteract Lele's Terrain moreso than other tapus. I think Tapu Lele is actually better than Gardevoir now, due to the Pixilate nerf, and the fact that Tapu Lele can hold different items to alter its power or speed, making it a lot more unpredictable
As much as I get why people would support a Rachi, Hoopa or Lele ban, I think removing any of those would do more harm than good and cause a chain reaction of subsequent bans. I'm pretty indifferent to whether Azu is hypothetically banned, but in the current meta I think it's balanced. I just urge people to question if the mons they accuse of being broken really are, because once we start banning these mons we're probably not going to stop suspecting things for a while.
Jirachi: If there is a good reason to ban it, it's because it can be abused to give mons like Azu and DD Mence a free setup. It also isn't the glue mon it was in ORAS, since sleep redirection isn't really relevant anymore. However, banning Rachi probably will trigger a chain reaction where Lele might warrant a suspect. DAD1's post above pretty much summarizes my thoughts on why Lele is fine right now, but like you said Pocket Lele + Deo-A would be a lot tougher to deal with. Of course, Trick Room would still counteract Lele + Deo just fine, and there are other good steels like Celesteela, Aegislash and Metagross-Mega that can stand up to psyspam. In my mind, people would still be split on whether Lele was broke, but I think more people would be swayed in favour of a Lele ban if Rachi was gone. Banning Lele would further trigger a chain reaction, which I'll get to a bit further below.
Azumarill: Again, I'm pretty indifferent here. Some people love it, some people hate it. Banning it won't trigger any chain reactions, we'll just stop seeing bunnies on the battlefield. Fwiw, it could become a lot tougher to deal with if we ban Lele - again I'll get to that a bit further down.
Hoopa-U / Hyperspace Moves: Now that we're in a more specially-oriented meta, Hoopa-U is shining a lot more. QSNS's new bulk spread for Hoopa also makes it a lot thornier to take down, wheras in ORAS iirc people mostly used Life Orb and maybe Sash. It is also (aside from Kyurem-Black and some Megas) the only 700 BST mon in doubles that hasn't been deemed an Uber, and hey maybe it's time to revisit that.
However, Hoopa also keeps a lot of things balanced in DOU that would otherwise be broken. First of all let's talk about Trick Room. Aside from Jirachi, Trick Room is the only other real thing keeping psyspam hyper offense in check. The problem with Trick Room on its own, however, is that 4 turns under Trick Room are really only 2 when your opponent simply spams protect to stall out turns. Hoopa fixes that by being able to penetrate Protect, and while some people view that as what makes Hoopa broke it's also a force that makes Trick Room viable. Tbh I'm not sure if I've seen a decent Trick Room team this gen that didn't have Hoopa on it. Of course, Hoopa does enable Hypnozong to be more threatening to the point that some people want Zonger banned, but I think Hoopa holds together more of the meta than it breaks. In the first months of this meta DOU was pretty HO-oriented until people realized how good Hoopa-U and Trick Room could be.
Tapu Lele: As I mentioned above, banning Rachi could prompt a chain reaction where more people would want to suspect Lele. In some ways, banning Lele wouldn't be that bad for the meta...the meta would just experience a paradigm shift where priority moves once again rule. Sure, Kang got nerfed a bit and Talonflame isn't going to be relevant anymore, but otherwise the meta would be ORAS again but with terrain wars and a few new mons. If you liked ORAS better, this might be a step in the right direction. If you thought ORAS was stale by the end of it, banning Lele is the quickest way to make SM feel stale all over again.
However, banning Lele would start another big chain reaction where we once again would have to look at banning things. Azumarill would once again become virtually unstoppable after it sets up, and if people already want it banned in SM I guarantee it's only going to become more controversial without Lele in the meta. Prankster Thundurus would also become relevant again, and we'd be back to seeing teams that spam T-Wave and Swagger, which means we'd have to revisit if Swagger warrants a ban. Really, THALK as a whole would become a prominant team style again, except for maybe Amoonguss which would still suffer because of Koko and Fini.
