Some Quick Theorymons:
Kingambit/Glastrier
Supreme Overlord
100/171/152/76/112/57
Incredibly strong, incredibly bulky, very slow for this metagame but the bulk seems insane along with the attack stat. Supreme Overlord is likely preferable over Chilling Neigh. Glastrier has some of the best donor stats for any slow, bulky physical attacker (+0/36/32/16/27/7)
Dragapult/Barraskewda
Infiltrator
88/150/90/115/87/176
Fastest mon possible without using Electrode as a donor mon, since Skewda over Electrode gives 150 > 132 Attack. Might not be good into the very splashable Baxcalibur and Kingambit fusions however as it's very frail and weak to both Ice Shard and Sucker Punch.
Dragapult/Meloetta
Serene Grace
88/139/94/132/107/164
Alternatively, Meloetta provides Dragapult with 40% chance Sp.Def drops from Shadow Ball, making special sets very hard to safely switch in to. You could even run Dragon Breath as a 60% para chance move, although this is very weak and only functions to hinder repeated switch ins further.
Dragapult/Bruxish
Dazzling
88/146/92/117/92/165
If prio on Gambit and Bax becomes way too meta for Pult and you REALLY want to counterteam... (Not Recommended). Tsareena provides 7 more bulk in both stats and 4 more Attack, but is 5 Speed slower.
Ceruledge/Kleavor
Sharpness
75/158/103/71/117/106
This might be too slow for a relatively frail offensive mon in this metagame (lol) but Bitter Blade is boosted by Sharpness to 135BP while Ceruledge's lack of effective Ghost STAB outside of Shadow Sneak can be somewhat patched up with its access to Night Slash, Psycho Cut (both 105BP) and X-Scissor (120BP). Gallade can be used instead to trade 2 Attack, 7 Defence and 1 Speed for 11 more SpDef.
Dragonite/Lokix
Tinted Lens
91/159/114/113/113/103
Losing Multiscale is tough, but Tinted Lens makes Extreme Speed unresisted outside of Ghost types so this could be a pretty potent cleaner able to muscle through weakened Steel-Types. Likely still loses to Tusk though.
Heatran/Tornadus-Therian
Regenerator
91/115/126/157/128/107
Everyone loves Levitate Heatran, but what about faster, stronger Regenerator Heatran? This mon is already strong and would likely appreciate even more power along with a reliable recovery method.
Zapdos/Gholdengo
Good as Gold
90/105/108/158/112/121
Gholdengo gives Zapdos even better defensive stats, even stronger offences and allows it to freely switch in against mons like Corviknight trying to Defog, while also allowing Zapdos to sit on bulkier mons who would otherwise wall and Toxic it like Clodsire.
Iron Moth/Indeedee
Psychic Surge
80/86/73/166/133/133
Lele at home... Should block a lot of the Kingambit spam and destroy them with Fire type moves, while Psychic Surge powers up an already strong Psychic.
Garchomp/Greninja
Battle Bond
108/153/111/105/102/132
The ultimate DD Chomp LARP, Gren gives a lot of speed, decent Attack and Sp.Atk and most importantly Battle Bond (Assuming this remains legal). A mixed set could even be run to deal with physically bulky mons like Great Tusk.
Hariyama/Diancie
Guts/Sheer Force
144/145/97/65/97/62
Basically Iron Hands with better Sp.Def and two very strong abilities, may be able to Belly Drum sweep through bulkier teams and deal with would-be revenge killers with Bullet Punch but might find it hard to set up against the very strong offensive mons in the format. Could deal with Kingambit decently although not resisting Iron Head hurts.
Iron Valiant/Iron Moth
Quark Drive
74/147/105/155/87/143
This just functions as a normal Valiant but on major drugs, with Booster Energy you outspeed every non-scarfer bar Booster Valiant/Scream Tail who you tie with.
Great Tusk/Goodra-Hisui
Sap Sipper
115/156/156/80/90/102
Goodra-Hisui bolsters Tusk's already great Attack and Defence while giving it an immunity to one of its weaknesses with Sap Sipper and patching up its low SpDef to a respectable 90.
Roaring Moon/Avalugg-Hisui
Strong Jaw
105/170/117/63/110/128
Although Avalugg doesn't boost Moon's speed, 120BP Crunch/Jaw Lock along with a titanic 170 Attack stat will be hitting hard and the natural physical bulk is appreciated too.
Slither Wing/Landorus-Therian
Intimidate
85/171/101/111/125/103
Sp.Def Buzzwole? Very strong pivot with Intimidate and access to immediate recovery in Morning Sun, can threaten out frailer sweepers with First Impression and use strong STAB U-turns to bring in teammates.
Slowking-Galar/Diancie
Regenerator
95/90/117/135/147/42
Another instance of Diancie's great donor stats (+0/25/37/25/37/12) making a bulkier mon itself but better.
Typhlosion-Hisui/Torkoal
Drought
73/105/113/140/102/100
Choice Scarf/Specs Eruption in the sun... If you want a real pokemon use Moth/Torkoal but this has at least 10x more swag. Hisuian Typhlosion is appreciated for the secondary STAB vs the inevitable Well-Baked Body Corviknights and other Fire immunities.
Walking Wake/Pelipper
Drizzle
99/95/116/148/100/125
Same concept but with a worse donor and a better base mon, this doesn't really have any switch ins outside of bulky resists and immunities. Unfortunately loses Flamethrower for coverage but Hurricane becomes 100% accurate and hits any Grass types that resist you, so arguably does a better job aside from against opposing sun.
Gholdengo/Heatran
Good as Gold/Flash Fire
87/82/121/165/117/103
Giving it better bulk and overpowering strength, Gholdengo can also gain a Fire immunity in Flash Fire but may appreciate keeping Good as Gold more (this ability is busted).
Toedscruel/Electrode
Static/Soundproof
80/82/82/100/140/137
Marginally stronger and bulkier Toedscruel? I sleep. Base 137 Speed Spore with no Mycelium Might? I also sleep.... Use with a booster mon like Valiant, Scream Tail or Moth for even more horrible results, and remember this guy gets Knock and Spin too. Depends on if this turns out to be as good in practice but wouldn't be surprised to see this limited in future.
Meowscarada/Rillaboom
Grassy Surge
76/141/92/96/87/144
A fast and strong team support option, able to set terrain and having access to Knock Off and U-turn along with a nuke in terrain-boosted Flower Trick. Meow's high base speed allows it to outspeed many other fusions even with a lower speed donor like Rillaboom.
These are just a bunch of ideas for sets, nothing here is tested; Really cool OM and looking forward to seeing how the meta develops/what gets looked at or banned down the line!