I'm missing the Special Guests from Strixhaven:
https://scryfall.com/search?q=e%3Aspg+cn%E2%89%A5149+cn%E2%89%A4158+game%3Apaper&unique=prints&as=grid&order=set

I'm seeing the RCQ promos for this season missing:
https://scryfall.com/card/slp/50/ancient-stirrings
https://scryfall.com/card/slp/51/sowing-mycospawn

The double-sided tokens from Secrets of Strixhaven. While the tokens are displayed on Scryfall, the double-sided element of them is not, hope this still counts:
https://scryfall.com/sets/tsos
The tokens I own are:
2x Copy (0001) // Spirit (0010)
1x Elemental (0002) // Fractal (0004)
2x Elemental (0002) // Treasure (0012)
3x Elemental (0003) // Treasure (0012)
1x Fractal (0004) // Inkling (0006)
1x Fractal (0004) // Pest (0008)
3x Fractal (0005) // Pest (0009)
1x Inkling (0006) // Spirit (0010)
2x Inkling (0007) // Spirit (0011)

Looks like the SLD MagicCon 2026/MagicFest 2026 Soul's Attendant is missing. Linked the Scryfall below:
https://scryfall.com/card/sld/7147/souls-attendant

SOS Art Series

SOS Double-sided tokens

SOC Double-sided tokens

ECL Double-faced card tokens