Heya! I'm making this post to talk about curious (and plausible) influences to actual cards, made by "acorn" cards made by third parties. The places i looked at is the site The Magic Librarities.
A nice touch seen in the Librarities are cards, real or virtual, not found in other places like Scryfall, beings originals made by magazines, or specific tokens for a sole card effect. Sadly, barelly, if any, are made in such old way. An example of influence is the mechanic of "random", used on the MicroPose Computer Game and the japanese Sega Dreamcast game, now usedon Magic Arena and Alchemy. Fun, but can only work properly on virtual platform. Other, not really unexpected, is the magazine The Duelist Fantasy Cards, with give some "extra unglued" cards, called Extra Pulled, which follows the un-set logic (or lack) (its seems to be also the source of the legendary Throat Wolf and the Firstest Strike).
But the cream is the InQuest, with loads of random themes (serious and jokes), mechanics and even actual MTG artists as guests. First, the idea of the color Purple since 1997; Then, what is a normal process to most franchises, to use another franchise but in MTG format, with Conan the Barbarian and Tarzan, and eventually the official Dungeons and Dragons and Lord of the Rings; IQ's System Shock comes with a "Command Zone" card with the type Quest, to fulfil a Task and get your Reward; IQ's Altered States is what some cards would look if they where other colors and effects, like a precessor for the MTG's Planar Chaos; IQ's space adventure, M:TG 3K, also comes with new names for types, like technology for artifact, and starship for a proto-vehicle with the Crew mechanic, but no explanation what should it be or the other mechanic, "Manpower".
To almost finish, there is a french group fan card maker, who made a set themselves, "Unofficial Cards", and two ideas shows quite a similar concept to the official MTG, latter on: a cycle of Eater (Dévoreur) type creature, being base for the Odyssey's Lhurgoyfs; and the Keeper of the Seven Circles, black legendary beings (seven hells?) who will look what could be the MTG's Seven Steel Thanes, also black legendaries.
An extra: a nice cycle to finish is the MMMM-cost sorceries about the life and family of Richard Garfield Jr. (Ph.D.): the white Proposal (unknown art), blue Fraternal Exaltation, red Phoenix Heart, and green Splendid Genesis (hope the black is not his funeral...).
There are many notes there regarding what cards eventually became of this (some in Un sets). My favorite "holy ***** this would be broken now" cards are Mana Beetle (Buried Alive is just add nine mana okay), Mana Magnet, all the Antimana cards (they'd be Better Than Black Lotus with just two of the right duals in play and keep getting better), Stunt Double (possible Ninjitsu idea and okay Griselbrand turn two without acceleration), Unnatural Selection (imagine this with the Mirage tutors), Lunar Ring, and Moonstone which became Heartstone.