Most of the things have already been said, but since I saw you mentioned this:
Here is a list of all Pikachu artworks that were never released in English (although some were released in Korean/Thai/Indonesian):
- WB Pikachu with Jungle background (Toyota Campaign promo / Bulbasaur Intro Pack)
- Birthday Pikachu (Natta Wake)
- Flying Pikachu (Nippon Airlines “Get in a Jet! Double Chance Campaign”
- Flying Pikachu (Nippon Airlines “Everyone’s Happy Campaign”
- Surfing Pikachu (JR Trainer Rally)
- Pikachu World Collection 2010 Blue background
- Pikachu World Collection 2010 Green background
- XY-P Art Academy gogotake (Farmer)
- XY-P Art Academy pikamania (Business)
- XY-P Art Academy Mame Akimaru (Space)
- XY-P Art Academy Y. Fujishima (Red Raincoat)
- XY-P Art Academy Sayaka Maruyama (Orange Raincoat)
- XY-P Art Academy Ginga (Pikachu Emotions)
- Ooyama’s Pikachu (Vending Machine Series 3 Green)
- 010/018 McDonald’s Pokémon-e Minimum Pack
- 004/009 PokéPark Premium Files - PokéPart Forest
- 003/009 11th Movie Commemoration Random Pack
- 0167/047 Everyone’s Exciting Battle
- 005/026 Ash vs Team Rocket Deck Kit - Ash Half Deck (also released in Korean)
- 019/053 Sword & Shield Family Pokémon Card Game (released as both non-holo and holo)
- SNAP Photo Contest 1999
- 004/P promo
- 021/P promo
- 038/P promo
- 044/P promo
- 043/PCG-P promo
- 098/DP-P promo
- 099/DP-P promo
- 100/DP-P promo
- 101/DP-P promo
- 102/DP-P promo
- 043/DPt-P promo
- 079/L-P promo
- 120/BW-P promo
- 151/BW-P promo (also released in Korean)
- 229/BW-P promo
- 050/XY-P promo
- 056/XY-P promo
- 068/XY-P promo
- 069/XY-P promo
- 070/XY-P promo
- 090/XY-P promo
- 094/XY-P promo
- 095/XY-P promo
- 096/XY-P promo
- 097/XY-P promo
- 098/XY-P promo
- 099/XY-P promo
- 150/XY-P promo
- 151/XY-P promo
- 175/XY-P promo (available both with and without Event Organizer stamp)
- 203/XY-P promo
- 204/XY-P promo
- 207/XY-P promo
- 208/XY-P promo
- 221/XY-P promo
- 230/XY-P promo
- 231/XY-P promo
- 274/XY-P promo
- 275/XY-P promo
- 279/XY-P promo
- 293/XY-P promo
- 294/XY-P promo
- 295/XY-P promo
- 296/XY-P promo
- XY-P Soccer promo 1
- XY-P Soccer promo 2
- XY-P Pokémon With YOU (Wagon) promo
- XY-P Fujita Nikoru promo
- XY-P 20th Anniversary Festa Tournament promo
- 005/SM-P promo
- 013/SM-P promo
- 014/SM-P promo
- 037/SM-P promo
- 038/SM-P promo
- 055/SM-P promo
- 061/SM-P promo
- 061/SM-P + 068/SM-P promos (same artwork, one with and one without stamp and different promo numbers)
- 088/SM-P promo
- 191/SM-P promo (also released in Korean)
- 192/SM-P promo (also released in Korean)
- 193/SM-P promo (also released in Korean)
- 194/SM-P promo (also released in Korean)
- 195/SM-P promo (also released in Korean)
- 196/SM-P promo (also released in Korean)
- 197/SM-P promo (also released in Korean)
- 200/SM-P promo (also released in Korean)
- 208/SM-P promo
- 209/SM-P promo
- 210/SM-P promo
- 211/SM-P promo
- 224/SM-P promo
- 227/SM-P promo (also released in Korean, Thai, and Indonesian)
- 270/SM-P promo
- 280/SM-P promo
- 281/SM-P promo
- 282/SM-P promo
- 283/SM-P promo
- 288/SM-P promo
- 325/SM-P promo
- 369/SM-P promo
- 392/SM-P promo
- 400/SM-P promo
- 407/SM-P promo
- 002/S-P promo
- 036/S-P promo
- 099/S-P promo
- 105/S-P promo (also released in Korean)
- 144/S-P promo
- 208/S-P promo
- 227/S-P promo
- 323/S-P promo
- [Jumbo] Pikachu, Jigglypuff, and Clefairy CoroCoro Jumo promo
- [Jumbo] Pikachu Summer’s Vacation CoroCoro Jumbo promo
- [Jumbo] Pikachu & Pichu CoroCoro Jumbo promo
- [Jumbo] Happy Birthday Pikachu Jumbo promo
- [Jumbo] Yokohama Captain’s Pikachu Jumbo promo
- [Jumbo] Birthday Campaign Pikachu 2010 Jumbo promo
- [Jumbo] Moving of Nagoya Pikachu Jumbo promo
- [Jumbo] BEAMS x Pokémon Pikachu Jumbo promo
- [Jumbo] Birthday Present Pikachu Jumbo promo
- [Teach Set] Teach Set July 2011 Pikachu
Additional unreleased Japanese exclusive artworks:
- Pikachu & Ishihara GX (Ishihara’s 61st birthday gift)
- E3 2001 Sample Set
- Japanese Press Conference 2001 Sample Set (two different versions)
- Japanese Space World 2001 Sample Set
- 1996 Prototype card
So a total of 126 different Pikachu TCG artworks that exist in Japanese and were never released in English.
PS: There is also:
- One Spanish exclusive Pikachu artwork: Art Academy Lily-Chan (Dress)
- One Korean/Thai/Indonesian exclusive Pikachu artwork: 001/SM-P / 004/SM-P / 009/SM-P promo in their respective languages
- And just a couple for which the reverse is true (released in English and other languages, but not in Japanese):
- XY174 promo (also released in German, French, Italian, Spanish, Portuguese, Korean, and even Russian, but not in Japanese)
- SWSH074 Special Delivery Pikachu promo
- 5/5 Poké Card Creator Pack
- Pikachu World Collection 2010 (USA flag)
- Art Academy Meli (Chef)
- Art Academy Rhivern (Cooking)
- Art Academy Xime (Dancing)
- Art Academy Ashley (Kabuki)
- Art Academy Me! (Queen of Hearts)
For pictures of all these cards (except the unofficial releases, which were mentioned separately), I can refer to my Japanese ‘Pikachu around the World’ collection post.
Greetz,
Quuador