Interesting team. I like how Breloom synergizes with Registeel, too. I think Snorlax and Raikou are decent partners. They all overlap on Defensive Celebi and have complementary relationships against Gengar. And Starmie is a nice choice, too, offering a needed bulky Water-type and Spin support, while also contributing pressuring Gengar and improving the speed of the team.
I have a few points of concern. Breloom is completely walled by Ghost-types without any means of breaking through them. Registeel is also weak to Taunt Gengar, and Starmie is neither bulky enough to Spin several times in the game (or contribute enough defensively) nor supported enough to guaranteed Spin against Ghost-types. Aerodactyl is also lacking support into bulky Water-types and non-Claydol Ground-types, and I don't think it gives much back to the team in terms of real backbone, although I can understand the defensive utility its speed brings.
I think it might be improved by slotting a ground type somewhere to help against physical Tyranitar. The simplest way to improve this team in my eyes would likely be to add Claydol in place of Aerodactyl and running a bulkier Starmie with Recover instead of Thunderbolt (or Ice Beam?). I would also recommend some more progressive set changes, such as making Registeel somewhat more offensive. While you don't completely solve the problem of Breloom being completely blanked by Gengar and suffering with Celebi, Claydol adds an extra Gengar and Celei lure. Its Explosion should help.
I've made this edit to maintain the integrity of most of the team, but it's not perfect:
https://pokepast.es/dce8cfe76451aa0e
View attachment 818177
Breloom is faster than 252 Spe neutral Tyranitar. I gave it a bit of bulk, since it doesn't really need so much speed when it's running this set. I would recommend a different set. In fact, I prefer HP Ghost on Breloom and Stun Spore, which allow it to slow down opposing Gengar and Salamence while offering more into Claydol. If you keep this set, I would recommend reserving Spore early and aiming to snowball momentum. I might also suggest looking to slot Pursuit Tyranitar to support it, but finding a slot that doesn't sacrifice something important is difficult.
Opposing Claydol and Celebi are an annoyance for this team, so it might make sense to consider Shadow Ball on your own Claydol -- in fact, I highly recommend it, should you consider a different set on Breloom. Claydol's EVs here guarantee that Explosion OHKO 252 HP / 0 Def Zapdos from full and offers better rolls into Blissey. Explosion is also intended to lure Celebi. I think this slot is a huge value add here, so I strongly encourage making this change.
Starmie has Recover and bulk to help it beat Metagross and sit on Swampert. I think this team can afford a slower, bulkier Starmie because you have Claydol to help against Gengar. Ice Beam helps against Salamence defensively, which is a problem for Breloom. It also offers additional pressure into opposing Zapdos and Celebi, who can trouble some members of the team. Ordinarily, I would use Thunderbolt instead of Ice Beam to better pressure Suicune, but seeing as you have multiple mons that pressure or abuse Suicune and threaten to trade with it, I think you might appreciate the extra pressure and endgame option of a multi-coverage Starmie with Ice Beam. You can consider a Calm set to better duel Pursuit Tyranitar and Gengar, or alternative EVs, but I think Recover and high HP investment are the way to go.
I've made Registeel a mixed attacker set. The attack investment offers some rough rolls into Blissey, where it can OHKO 0 HP / 252+ Def Blissey. Superpower also has improved rolls against Tyranitar, which should greatly help Raikou and Snorlax. Explosion should have better rolls against Celebi and Claydol as well. HP Grass is for Swampert, obviously, who is easier to switch into for Starmie now, but also helps Raikou out a bit. With multiple special checks, including 2.5 Zapdos switch-ins besides Registeel, I think Registeel is freed to run a more targetted and progressive set. An alternative set might be Toxic / Thunder Wave / Seismic Toss / Explosion or a Curse set with HP Ghost, where Toxic allows Registeel to pressure Claydol and Swampert for its teammates better and Curse + HP Ghost allows it to better land attacks on Gengar. My one major point of concern with slotting a more conventional Registeel set is that it struggles to meaningfully chunk Gengar. Thunder is a small improvement in that regard. Otherwise, more actively pressuring Tyranitar and Swampert are huge value adds for Snorlax and Raikou, since switching into them multiple times a game is undesirable, and these moves better fit the tempo of the team. I think mixed Steel makes the most sense here, though.
Snorlax is EV'd to tank 2 +1 252+ SpA Hydro Pumps from Suicune and withstands 2x Dugtrio EQs. Brick Break is an interesting try on your part, and it's a good option on this team, too. I chose to drop Brick Break and use a more offensive Registeel with fighting coverage instead, since I think Snorlax is better at trading up this way. Earthquake instead of Self-Destruct would allow it to play more slowly against the likes of Metagross and Tyranitar, and even to bully Jirachi. However, I felt that it might be better to have Self-Destruct for Bold Suicune. I would otherwise swap out Body Slam for Double-Edge, since being able to force Suicune to Rest could be useful, but since I dropped Roar from Raikou and there is no way of guaranteeing Snorlax stays at high enough HP to avoid being trapped by Dugtrio -- one of Bold Suicune's most common parters -- I figured Self-Destruct might be more consistent. This also allows Snorlax to trade with opposing defensive Celebi to unlock the team's offense and opposing Metagross, which improves the defensive backbone of the team. In reality, you can justify a number of different coverage options next to Curse / Shadow Ball / STAB on Snorlax, and many of the justifications here overlap with other Pokemon on the team, but redundancy does unlock the heightts of offensive synergy, provided you don't tunnel vision.
Raikou offers a Gengar switch-in. Natural Cure on Starmie does, too, given a Calm nature. Raikou also offers a strong set-up threat that can threaten opposing offense quite nicely. I've elected to slot Substitute to help beat Calm Mind Blissey, Dugtrio, and Toxic Jirachi. Crunch allows it to KO Gengar from higher HP ranges and threaten Claydol and Celebi, which is important here. Of course, you can't always guarantee it has a Sub up against Dugtrio, and you miss HP Grass against Swampert or Ice against Flygon -- although the rest of the team should be good against Flygon teams -- and you don't really benefit from Substitute against Seismic Toss Blissey. I pushed the speed to the max to offer revenge killing, and maxed bulk to offer better utility on the special defensive end. This might be over doing it, though. See what you like. There are trade offs to whatever Raikou set you use, and I'm not 100% sure how to optimize this mon without some way of beating Dugtrio. Without P2 or Agility Pass Zapdos, you typically have to look elsewhere on the team -- like with DD Mence, for example. Dugtrio is a real problem that I don't know how to deal with here, but Sub spam should help.
Earlier, I mentioned swapping in a Pursuit Tyranitar. This might have to be instead of Claydol (or Raikou) and offers some funtional similarities with upsides, but you then have no Earthquake-immune Pokemon. Zapdos or Salamence instead of Raikou could help this problem, but the team will probably lack firepower in that event -- although a mixed attacking Salamence with Wish instead of HP Grass would offer much desired clerical support and free up other sets. Celebi could also be used instead of Raikou to similar effect.
Here's a similar team from Johnald with Registeel and Breloom that impressed me:
https://pokepast.es/d38c9f4f5dd5e2ab
Here are a couple of rough (untested) sketches I have lying around in my builder of similar concepts These two are older:
https://pokepast.es/b3dd9770a7bb507f
https://pokepast.es/9dede51be0687a19
Here's one I built inspired by
this game from mielke (Check
this gamee out, too, with Hera>Breloom):
https://pokepast.es/475ca4f5ac6cc173
Hope this helps you find something you like. I think finding a way to fit Pursuit Tar or Wish Mence could help fix some of the problems, but I think you've got a decent squad, so long as you find a way to keep a floater, replace Aero, and keep the team solid against Gengar. Happy building!