Games:
-The fact some of my early playthroughs were done via emulation, because now I can't transfer those Pokémon and have them with me. I really miss my Emerald team; I tried to recreate it while playing (an original copy of) Alpha Sapphire the first time, but it just wasn't the same.
-Some of my starter choices for first playthroughs. I picked Snivy in both Black and Black 2, but looking back, I wish I had gone with Oshawott. The anime and the PLA merch drive finally made me see just how precious it was, and even back then I didn't feel as attached to my Snivy/Servine/Serperior as I was to the rest of my team. ;_; I also regret having picked Fennekin over Froakie in Y, but this one is more subdued; my Braixen is my baby, but I just couldn't bring myself to fully evolve her and it's a miracle that game was so easy that I could cheese it with an Everstoned starter. It also gives me the chance to breed for a shiny Froakie sometime in the future, so I can have a black Greninja!
-Having released a Zoroark that a former friend traded to me like 11 years ago, simply because I didn't want to see their name anywhere in my game. It's not so much the act of scrubbing the OT's name (and the Zoroark's admittedly horrible nickname) that I regret, but of letting the Zoroark go, because I had put a lot of effort into training him. Sadly, I couldn't bond with him fully, and in hindsight, now I know why. Wherever he is, I hope he's enjoying being out and about in the Alola region, and I hope the wild Pokémon there call him a better name than the one he had.
-Not having pre-ordered the DLCs for Sword and Shield. I didn't trust Game Freak to do a good job on them, so I decided to wait until both packs were released. By the time they did come out... the situation with currency exchange in Argentina had become so messed up that I could not bring myself to buy them even though I did (and still do) want to.
Merch:
-Having bought that Gengar pendant I got like 5 years ago from a local convention. I swear it's cursed, because bad things happen every time I wear it - and now I'm afraid of getting rid of it because I know, I just know it will find its way back to me. It's gotten so bad, it has affected my liking of Gengar in general.
-Conversely, not having bought a Soul Badge pin alongside my Soul Badge pendant. Sure, they were both the same shoddy quality, but at least I would have had a nicer-looking Badge now, because my pendant is looking banged up and awful. (And again, I'm afraid of getting rid of that pendant because I probably won't find another like it ever again. Much like Koga himself, it's one of a kind.)