UW - Spirits
WB - Vampires, Lifegain Subtheme
WR - Humans, Keywords
WG - Humans, ft. +1/+1 Counters
UB - Zombies
UR - Spellslinger
UG - Transform/Morph, Graveyard
BR - Vampires, Blood
BG - Graveyard, Toughness, Spiders
RG - Wolves

Jeskai Burning Vengeance: If you see an early Burning Vengeance, consider this archetype. The cube is filled with enablers and alternatives for BV like The Lost and the Damned, Thalia's Geistcaller, and Vega, the Watcher.

Temur Transform: If you're firmly in green and see a Mutagen Connoisseur, it may be a good idea to splash blue. This card is going to be an absolute powerhouse, and blue's instant-speed tricks allow you to flip your werewolves with ease.

W/G/x Counters: A few selesnya-adjacent uncommons lend themselves well to the +1/+1 counters archetype, like Sunrise Cavalier and Metropolis Angel. Since you're in green, splashing for these will usually be worth it.

Sultai Spider Spawning: Self-explanatory. If you're in the golgari archetype and see a Vilespawn Spider or a Prized Amalgam, it's an easy splash.

Esper Skies: If in UW and you see a Halo Forager or a Regal Bloodlord, that can be a good splash into a more grindy version of the Spirits deck.

Jund Tokens: If you're playing Spider Spawning or Blood tokens, it's often a great idea to merge the two archetypes thanks to their synergy together. This allows cards like Sanguine Brushstroke and Blood Artist to really shine in the blood shell when combined with a Spider Spawning.

Mardu Tokens: Turns out Hidden Stockpile fits into the Blood deck pretty well. This card can do ridiculous things, as can Wernog. White also has some vampires like Markov Waltzer to go with your lords.

Grixis Rise from the Tides: This card is pretty broken, especially with the zombie synergies floating around. Pack a deck with spells and this card will often win the game on the spot.

B/x/y Aristocrats: There are so many color combos for this. If you can land a Morbid Opportunist or a Blood Artist early, this might be good to look into.

House Rules:
All Werewolves use Day/Nightbound
Transformed creatures count as "face down" (Secret Plans)
Transformed Creatures on their back face count as Modified (Invigorating Hot Spring)