Lets theorymon because theorymonning is cool and definitely not dumb.
This post will primarily focus on Zacian-C. Quiver Dance ban is much less impactful in comparison but I will mention it if its important.
Zacian-C ban is kind of unique in that almost every other Pokemon in the tier benefits from its departure. Other offensive mons did not appreciate its revenge-killing potential, as well as fantastic typing and bulk that forces very specific moves to nail it (which could get blocked by the right immunity ability. Physically defensive mons can slack a bit with Magical Torque confusions and not dedicate moves to hit it. Specially defensive mons don't have to deal with utility Zacian-C sets switching in and exerting great Pressure. Basically, almost everyone else in the meta benefits, albeit to different degrees. Since that is a lot of mons, this post will just consider the potentially viable fast offensive mons, with "fast" meaning outspeeding Arceus.
While Zacian-C existed, a lot of these mons were basically unviable, because Zacian-C outclassed them, checked them, forced suboptimal moves etc.
200 Regieleki loses a big niche in being the only remotely viable mon that outsped Zacian-C. It still remains the absolute fastest mon in the meta without even a speed boosting nature, outspeeding Neutral 116 or Positive 102 +1 mons and Neutral 76 or Positive 65 +2 mons, giving it some revenge-killing potential. That being said, it's pretty weak outside of a potential Hadron boosted Rising Voltage, with stuff like unboosted Headlong Rush failing to 2HKO Dialga-O. Suicide leads face competition from the next mon, and Hyper Offense is questionable when Tera is unbanned.
148 Zacian @
could potentially see some use because in the end, its still a Zacian-C, albeit without the item or ability slot. Can see this running some utility sets, though the lack of an ability and inability to hold items like Covert Cloak can be very detrimental. Potentially reasonably viable since people are much less likely to prepare for this typing.
142 Dragapult is likely much better than Regieleki, possessing slightly more offense (Eleki runs boosting nature) and much better STAB types and moves. It's speed tier lets it revenge-kill the other Dragons in the meta, and the neutral coverage between its STABs lets it hit most of the meta reasonably well. Its damage output is also lacking a bit though, and it will struggle to force switches or take down bulky neutral targets without good prediction. A suicide lead set can compete with Regieleki due to access to Curse.
138 Hero-forme Zacian saw exactly 0 usage before due to Species Clause, but now it can be used viably. It has the same good Fairy typing, outspeeds the crowded 135 tier, and has good matchups into all of them. Defensive sets can see similar stuff to Arceus-Fairy, with noticeably worse bulk but better speed and not consuming the slot. Offensive utility sets can also be reasonable, as Fairy resists might decline, and Pixi Pop Bomb will still hit like a truck. Definitely see this as a reasonably good mon.
138 This mon is probably just outclassed by Koraidon, who is far more threatening to Ghost-types with its useful secondary type. The lack of Dragon-type doesn't even help since you don't really want to deal with any of the stuff in the 135 tier anyways (or Dragons in general) so its only for Chien Pao?
136 Iron Bundle has theoretically perfect neutral coverage if it runs Freeze-Dry (Specs does 2HKO Waterceus) which makes Refrigerate potentially appealing since Water STAB hits most Ice resists now that Zacian-C is gone. However, Scales Dialga-O is still a difficult wall to bypass, with not even Tera Ice 2HKOing from full. Can see this being used.
135 Nice STABs that hit 3 of the common FC walls from Zac-C meta (Ghostceus, Groudon, Eternatus) while having a neutral matchup into Miraidon without Tera. Highly appreciates no Zac-C. Needs Collision Course to nail FC Slaking but can see some use.
135 Can spam Fairy moves without real resists since Ghost STAB threatens all of them. Specs and Judgment sets both seem solid. Extra power compared to the aforementioned Iron Bundle lets it brute force through Dialga-O with Tera slightly easier.
135 The strongest mon in this list by attack. Offensive sets can take down moderately chipped FC Ghostceus with Glaive Rush. STABs hit enough stuff in the meta to allow it to dedicate a move for Fairies. Defensive sets can also be interesting now that physically Fairy moves are practically extinct, with a great typing to resist V-create while not being weak to other common moves, and the Fighting type helps against Population Bomb users.
135 Doesn't really need much discussion. We know how this mon was already strong and now its best revenge-killer is gone. Though, there can be more freedom to pack special walls.
130 Etern is still going to be a great defensive pick, with a nice typing and useful Toxic immunity. Offensive sets sees some positive change too with Zacian-C gone, though Etern was a very good QD user. We might see Poison STAB being run now.
130 Mewtwo differentiates itself from the other special attackers through its STAB Lumina Crash, which gives it a way of breaking past traditional special walls. It also has a reasonable 110 attack for potential mixed sets. However, it will still struggle with stuff like Dialga-O and notably Ting-Lu, who has great mixed bulk and is immune to Psychic. M2 also has a really strong Future Sight to support physical teammates potentially.
130 Ghost horse probably won't be very easily justified over Flutter Mane. The Fairy STAB is very useful and missing out on it for a bit stronger Ghost STABs doesn't feel worth it at all.
128 The fastest Normal Population Bomb user that is also hopeless walled by Ghosts?
128 Zama-C defensively got a small bump with less Magical Torques to worry about, and its probably one of the best Slaking switch-ins. I still don't really see defensive sets being good though with a Fire and Ground weakness, but utility sets can be interesting, similar to Zacian-C. That being said, neither of is STABs are very good, Fighting has an immunity while Steel is resisted by big threats like Miraidon and Palkia.
123 You know, if Brute Bonnet (or Zarude) can see offensive success, Meowscarada trades power for the speed jump on Arceus. Dark Grass is actually a pretty nice offensive typing, with Dark blocking Prankster and bypassing Sap drops with Wicked Blow while Grass STAB hits Groudon and Dondozo really hard.
120 and friends
This is a lot of potentially viable mons, and I haven't even considered listing the sub 121 speed mons. While playtesting will show which of these are actually viable and remain used, there will still be a diverse pool of mons to build around (much more diverse than pre ban), and with also numerous defensive picks, there should be a lot of variety in this new meta (kind of reminds me of pre-DLC SS meta but with actual defensive mons). Interested to hear your opinions as the meta develops from arguably a fresh slate too so post your thoughts here or on disc.