BSS currently allows restricted Pokemon, which means it's probably a good idea to wait for the next series if you want to build a full team of non-legendaries. Unfortunately, I do not know what the next series will be, so I can answer only for series 12. Also note that the most viable unrestricted Pokemon are viable because they work well with restricted Pokemon, so this is another reason why a full non-legendary team is not the best idea right now.
source
Ditto
With Imposter, Ditto becomes one of the most effective revenge killers in the format, dealing with massive threats such as Zacian-C, Calyrex-S, offensive Eternatus, and a variety of weakened Dynamax sweepers such as Seismitoad and Dragapult.
Imposter has a special interaction with Zacian's Intrepid Sword, granting Ditto an Attack boost upon transforming. This can make weakening Zacian-C into KO range of +2 Behemoth Blade a serious win condition to consider.
Ditto is a fairly capable user of Dynamax depending on what it copies, so always consider that as another potential win condition. Dynamax also allows Ditto to either get around a poor Choicelock or prevent itself from locking its move for three turns if used turn one.
While any Pokemon appreciates Stealth Rock support, it's especially notable for Ditto, since Stealth Rock nullifies Focus Sash and makes revenge killing Calyrex-S much more reliable.
Consider benching Ditto if facing a dedicated stall team or Trick Room team, as it has major issues breaking through them.
While Zacian-C and Calyrex-S are excellent targets for Ditto to use for sweeping, make sure to pay attention to team preview, as well-crafted teams may have Pokemon in the back to specifically prevent Ditto sweeps. Examples include Quagsire and bulky Steel-types against Zacian-C and physically defensive Porygon2 and Mimikyu for Calyrex-S.
Focus Sash Ditto needs a way to prevent Stealth Rock to be most effective. Teams with Choice Scarf Kyogre have a particularly good use for Focus Sash Ditto, since Choice Scarf Kyogre will be less expected with Ditto on team preview. Taunt users such as Tapu Fini, Grimmsnarl, and Yveltal are also good options to consider.
When using Focus Sash Ditto, consider some forms of speed control to give Ditto a better chance at sweeping foes. Paralysis support is the easiest option to consider, with a wide variety of Pokemon to choose from such as Porygon2 and Chansey, but another option includes Trick + Lagging Tail from Prankster Grimmsnarl, Mimikyu, and Tapu Fini. The most extreme method is using Sticky Web support, but setters such as Galvantula and Shuckle make this strategy unfriendly to beginners.
A Relaxed nature and 0 Speed IVs give Ditto the best chance of winning in the rare mirror match.
source
Quagsire
Despite mediocre stats, Quagsire makes itself very important by checking one of the best Pokemon in the game, Zacian-C! This makes Quagsire a staple both on stall teams and on more balanced and offensive teams that desperately need a Zacian-C check.
Yawn is highly recommended, as it lets Quagsire pressure special attackers that switch in as long as Stealth Rock is up, and it can stop Necrozma-DM before its Dragon Dances get too out of control.
Earthquake lets Quagsire usually 3HKO Zacian-C on its own. It also usually 4HKOes defensive Necrozma-DM, though Sunsteel Strike, which ignores Unaware, and Weakness Policy make this a risky tactic.
Scald is very weak, but it prevents Landorus-T from totally walling Quagsire, and the 30% chance to burn is quite useful. The burn can conflict with Toxic and Yawn, however.
Quagsire gets access to the rare and useful Toxic, which is great for putting bulky switch-ins to Quagsire on a timer, such as Kyogre.
Protect can help stall out powerful Dynamax users such as Life Orb Landorus-T that can threaten to break through Quagsire, and it also has great synergy with Yawn and Leftovers.
Stockpile is an alternate way to protect Quagsire against powerful physical Dynamax users, turning a lot of moves that 2HKO into a 3HKO, such as Life Orb Landorus-T's Max Quake.
Pairing Blissey or Chansey alongside Quagsire is highly suggested, since most powerful special attackers easily KO it. Blissey in particular does well against Kyogre thanks to Utility Umbrella.
Although Quagsire is mostly used to check Zacian-C, Water Absorb is a surprise option that makes Quagsire a good Kyogre check instead. The surprise factor at Team Preview is the main value, though, as Gastrodon is a better Kyogre check otherwise. When playing against Quagsire, consider the possibility of Water Absorb at Team Preview if it is paired with a strong secondary Zacian-C check, such as Necrozma-DM.
source
Porygon2
The people on Smogon have not yet started writing the analysis, so here's a sample set.
Porygon2 @ eviolite
Ability: trace
EVs: 244 HP / 44 Def / 220 SpD
Sassy nature
- ice beam
- foul play
- recover
- speed swap
source
Ferrothorn
The people on Smogon have not yet started writing the analysis, so here's a sample set.
Ferrothorn @ leftovers
Ability: iron barbs
EVs: 252 HP / 4 Def / 252 SpD
Sassy nature
- leech seed
- protect
- gyro ball
- bullet seed
source
Cinderace
With its great Attack, Speed, and Libero, Cinderace finds itself in a commanding position in the BSS metagame.
Pyro Ball becomes G-Max Fireball under Gigantamax, a 160-Base Power attack that also nullifies abilities like Mimikyu's Disguise and Heatran's Flash Fire.
Bounce gives Cinderace access to Max Airstream, which makes it a very potent sweeper, having nearly unmatched Speed alongside its colorful movepool and Libero.
Sucker Punch may seem odd on a set revolving around Max Airstream, but it can be useful in situations where Cinderace doesn't have a boost, such as revenge killing.
Giga Impact provides Cinderace with an obscenely powerful Max Strike, which is useful for both its wide neutral coverage and the Speed drop, which lets it maintain the Speed advantage in Dynamax battles against Zapdos without relying on Max Airstream. Max Strike is also Cinderace's strongest option against Quagsire.
Gunk Shot grants valuable coverage against Fairy-types such as Xerneas and Tapu Fini.
Iron Head still covers Xerneas while giving Cinderace access to Max Steelspike for Defense boosts.
source
