@c011ect0r filler collection comes to mind as well: List of All Filler Card Variations (Blank Errors) - 04/18/25 Update
And I guess some of my collection sub-goals are fairly unique, like the first TCG card released in each language; or all raw, sealed, and graded variations of the Moltres promo. 
Here is everything I collect for Pokémon:
- Pikachu TCG cards in all languages and variations up to Feb. 27th, 2021 (25th Pokémon Day), as well as ALL English Pikachu TCG; the Base Set artwork reprints in all languages; and one of each non-English exclusive artwork, after that date.
- All English full art Supporter and Character Rare cards, as well as non-English exclusive artworks.
- Everything Seviper. All Seviper TCG cards in all languages, including cameos; as well as all non-TCG Seviper products (plushies; figures; non-TCG cards; stickers; posters; DVD covers; pogs/tazos; etc. etc.)
- The Moltres WotC #21 promo in all languages; raw, sealed, and graded.
- The BREAKthrough Swablu in all languages.
- The first TCG card released in every language.
- Ancient Mew sixtet in PSA-10.
Numbers 2, 3a (TCG), 5, and 7 are 100% complete. For number 1 I’m missing nine Pikachu cards (five Japanese; two German; two Spanish). 3b is a never-ending journey. 
For number 4 I’m only missing some graded foreign variants, and iff it exists the Danish sealed promo. For number 6 I just have to get the Latin American Spanish Caterpie non-holo from the Journey Together set.
Greetz,
Quuador