I'd like to see baby Pokemon appearing in the wild nice and early. They've been doing much better with this in the last several games, especially Black 2 & White 2 which had Elekid/Magby very early on, but there's plenty of others that haven't been easily obtainable to use in a normal playthrough, like Tyrogue, Pichu, Mantyke, Munchlax, Igglybuff & Cleffa. I'd actually add Smoochum & Mime Jr. to that list too, because while they were found in Glittering Cave in X&Y, they were low-leveled in hordes making them pretty unappealing to use.
Clamperl is another pretty good pick. Can't imagine many people have used it before as it was hard to even know how to evolve it in the original R&S and hasn't appeared much since. Maybe have them sitting on a beach and interactable? I like the idea of having more interactable Pokemon in general. Maybe have some Corphish, Krabby and Shellder on the beach as well? So many Water-types have been locked away behind a Fishing Rod.
Next up, Natu! You could only catch Xatu in X&Y&OR/AS, and only in the Friend Safari, Safari Zone and Mirage Island. It's a cool typing and a fitting Pokemon to find early on.
Hmm, what else? Gligar, Sneasel, Phanpy, Duskull, Shuppet, Spheal, Hippopotas & Snorunt have never been found early iirc. I'd also say Larvitar! I know it's a pseudo, but XY was the only time it's been able to be caught before beating the Elite 4, and it was in Terminus Cave, so still way too late to really use. Dratini has been relatively early before. Bagon appeared pretty early in XY, and Beldum was literally given away as a gift Pokemon in ORAS, so I don't think it being a pseudo should be an issue. If you could find it around the 4th gym, you wouldn't be evolving it into a Tyranitar until the league, anyway.