Lemme just add 5 to attack to my Garde sub since people have been pointing out how good it is.
New Stats: 77 / 70 / 95 / 130 / 110 / 34 | BST: 516 ( + 25 Def, + 25 SpD )
New Moves: Nasty Plot, Power Gem, Earth Power, Energy Ball
Reasoning: The reasoning for the defense boosts is simple: Trick Room setters generally need to be bulky in order to be successful. Think of it this way: there are plenty of Pokemon that can move first and hit hard without needing to set up a field condition for them ( much less one that only lasts 5 turns and needs to be set by a move rather than a switch-in ). Trick Room Pokemon hit hard, are bulky, and move first ( under Trick Room ). Nasty Plot is there to help with Trick Room's matchup against bulky teams; it may not always run it, but an "ultimate Trick Room abuser" should be able to fulfill this niche if need be. The extra STAB and coverage options help it be more of an immediate threat, an area in which I also think it was somewhat lacking.
Raticate-Alola-Totem
Abilities: Thick Fat / Gluttony / Extreme Bulk
New Stats: 112/111/50/40/50/87 (450 BST)
Custom Elements: Extreme Bulk: This Pokemon has 1.5x Attack while Gravity is active.
Justification: The main downside to Raticate-Alola-Totem was its bad Attack stat outside of Gravity and low speed tier. A significant increase to its Attack and a slight increase to Speed alleviates this problem, and Extreme Bulk is nerfed to compensate.
Base 71 at level 100 with 31 IVs, 252 EVs at +2: 482.
Base 111 at level 100 with 31 IVs, 252 EVs at +1: 481.
Pokemon: Flaaffy
Abilities: Natural Cure / Static / Fur Coat
Stats: 100 / 55 / 40 / 90 / 70 / 60
New Moves: Roost
Reasoning: Some of Flaaffy's major downfalls were it's lack of reliable recovery and inability to really benefit from Fluffy and it's interesting Electric/Flying typing. Fur Coat allows it to really become a good physical wall and pivot. I'm a little skeptical that this might be too tough to beat on the physical side and it might outclass Toucannon, let me know if you think there's a problem.+1 252+ Atk Swampert Liquidation vs. 248 HP / 252+ Def Eviolite Fur Coat Flaaffy: 76-91 (18.8 - 22.5%) -- guaranteed 4HKO after Stealth Rock
+2 252+ Atk Tapu Bulu Flare Blitz vs. 248 HP / 252+ Def Eviolite Fur Coat Flaaffy: 130-154 (32.2 - 38.2%) -- 2% chance to 2HKO after Stealth Rock
+2 252+ Atk Tapu Bulu Inferno Overdrive (190 BP) vs. 248 HP / 252+ Def Eviolite Fur Coat Flaaffy: 205-243 (50.8 - 60.2%) -- guaranteed 2HKO after Stealth Rock
252+ Atk Choice Band Metagross Zen Headbutt vs. 248 HP / 252+ Def Eviolite Fur Coat Flaaffy: 84-100 (20.8 - 24.8%) -- guaranteed 4HKO after Stealth Rock
252+ Atk Swampert-Mega Liquidation vs. 248 HP / 252+ Def Eviolite Fur Coat Flaaffy in Rain: 88-105 (21.8 - 26%) -- 5.1% chance to 3HKO after Stealth Rock
252+ Atk Mold Breaker Rhydon Stone Edge vs. 248 HP / 252+ Def Eviolite Flaaffy: 288-342 (71.4 - 84.8%) -- 68.8% chance to OHKO after Stealth Rock
252 SpA Nihilego Power Gem vs. 248 HP / 0 SpD Eviolite Flaaffy: 192-228 (47.6 - 56.5%) -- guaranteed 2HKO after Stealth Rock
252 SpA Nihilego Acid Downpour (175 BP) vs. 248 HP / 0 SpD Eviolite Flaaffy: 210-247 (52.1 - 61.2%) -- guaranteed 2HKO after Stealth Rock
252 SpA Unown Psychic vs. 248 HP / 0 SpD Eviolite Flaaffy: 135-159 (33.4 - 39.4%) -- 17.2% chance to 2HKO after Stealth Rock
252 SpA Xurkitree Thunderbolt vs. 248 HP / 0 SpD Eviolite Flaaffy in Electric Terrain: 171-202 (42.4 - 50.1%) -- guaranteed 2HKO after Stealth Rock
+1 252 Atk Lycanroc-Midnight Splintered Stormshards vs. 248 HP / 252+ Def Eviolite Fur Coat Flaaffy: 332-392 (82.3 - 97.2%) -- guaranteed OHKO after Stealth Rock
+1 252 Atk Lycanroc-Midnight Stone Edge vs. 248 HP / 252+ Def Eviolite Fur Coat Flaaffy: 176-210 (43.6 - 52.1%) -- guaranteed 2HKO after Stealth Rock
252 Atk Life Orb Raichu Ice Punch vs. 248 HP / 252+ Def Eviolite Fur Coat Flaaffy: 112-135 (27.7 - 33.4%) -- guaranteed 3HKO after Stealth Rock
As you can see, Flaaffy is hard to break physically if you aren't using super-effective attacks, but it's special bulk isn't anything special even after Eviolite, allowing it to be more easily killed.
Stats: 70 / 80 / 115 / 130 / 135 / 80 | BST: 610*
Ability: Levitate
New Moves: Moonlight, U-turn,
Reasoning: Gardevoir has a fantastic support movepool including moves like Will-o-Wisp, Thunder Wave, and Taunt, so going for bulk on the mega allows it to use these attributes in a way that neither clean slate nor real Gardevoir can. Levitate is a defensive ability that makes it a surefire Kommo-O switch in, something that's in short supply currently. U-turn is a solid option for base Gardevoir because it tends to exert a lot of pressure and yet be completely reliant on prediction, making U-turn a safe move for it to take advantage of.
New Stats: 85 / 85 / 90 / 120 / 95 / 65 | BST: 540
New moves: Quiver Dance, Heat Wave
Reasoning: I noticed there ins't a Quiver Dancer yet in the Clean Slate meta so here you go. I increased Porygon's bulk a bit too to make it easier to set up. Also Heat Wave because I hate Forretress. However, it still gets walled by either Nihilego, Lanturn, Druddigon, Forretress, Audino or Musharna depending on the coverage it chooses.
Metagross Speed is 75 so your Megagross should have 55 base Speed.
Name of the Pokemon: Metagross-Mega
Role(s): Offensive Trick Room setter, Trick Room abuser
Type: Psychic/Steel
Abilities: Sheer Force
Stat Distribution: 80/145/135/105/105/45 (+20 Atk, +30 Def, +40 SpA, +40 SpD, -20 Spe)
New Moves: Trick Room
Justification: Trick Room is not really seen in the metagame, so Metagross-Mega is a good addition to provide Trick Room support. It's balanced in that it doesn't gain much power from Mega Evolving, and it still lacks the coverage it wants.
edit: leonard (from big bang theory) Metagross-Mega is Psychic/Steel which is inconvenient for you