Dude- Dude Types are women and men who can blend in with 90% of the population but could really bring anything to the table. Orcs and halflings also go here, neat!
Goblin- Gob-Hogbog-Bugbear is the most natural pokevolution parallel in D&D but this tree also includes kobolds, gremlins, quicklings, anything gob-adjacent.
Fae- Elfs, pixies, sprites, pucks, wisps, so on.
Yucky- Cubes, oozes, puddings, Dragon Quest guys with faces, mimics
Bug- Carrion crawlers, umber hulks, rust monsters, stuff that crunches.
Arcane- All Magic-Users but also animate brooms and familiars and so on, or chimeras like the owlbear.
Rocky- Dwarfs, gnomes, piercers, ropers, neanderthals.
Elemental- The specific element doesn't matter, you're just infused with primordial power or made only of that.
Psionic- Berbalangs and all that stupid shit but also like whatever the Battlemind is supposed to have been...
Spooky- Skeletons, Vampires, Liches, Mummies, Witches, Werewolves, ghoulsghostsghasts anything very Halloweeny.
Mythic- Medusa, Coatls, Minotaurs, Goliaths, Angels, Demons, world religion and mythology creatures.
Zoo- For like regular animals and giant versions of the same.
Ninja- Shadows, invisible stalkers, and other things you can't see.
Lizard- Pretty much all reptile stuff, from yuanti to dragons, because D&D has a lot of lizard shit.
So we see how application of these principles to existing enemies is easy. Drow are Night Fae, Acererak is Spooky Arcane, Vecna is Spooky Mythic, Shadowfax is Zoo Mythic, Conan is Rocky Dude, Lava Children are Yucky Elemental, Venom is Bug Dude,dragons can be Arcane Lizards but Tiamat is obviously a Mythic Lizard, while regular old drakes are Zoo Lizards.
We'll arrange these on a diagram. That's right, this magic item requires a diagram. That's right, I'm doing all this for the sake of a single magic item. A Type is Weak (1/2 dmage) against the two types adjacent to it, and Strong (2x damage) against the two Types across from it that it can draw a straight line to on the star. Arrange these howeeeever it makes sense to you: in my example here, Rocky Types are Weak against Arcane and Elemental but Strong against Fae and Ninja. Our needs aren't as complicated as Pokemon's, though, so we can still have Arcane be Weak to Rocky without making things too complicated. I'll include a blank diagram I ganked from Google so you can lay out your own typings. Your actual layout doesn't matter a ton so long as everyone is on the same page about it.
OKAY SO HAVING DONE ALL THIS SHIT, this weapon is a magic sword that does +1 and overcomes magic resistance bla blah. ALSO it absorbs the essence of those it wounds. Write down two blanks and number them 1 and 2. Whenever you roll the highest number on this sword's damage die - be that a d6 d8 d12 whatever your DM decides - your sword will absorb the Typing of the creature you inflicted that damage on. You can decide not to take a target's Typing but you have five seconds to make up your mind or the effect fails.
Whenever you roll 20/d20 with this sword, you pick up a SECOND Type. This only changes when you roll a crit to hit and you don't get a choice in whether this Type changes or not, it just does. These modifiers stack and combine to mitigate or multiply bonuses.
If you have a player who spends all their time looking up obscure errata or arguing the wording of spells, give them one of these instead and it will take up all that time. Just try to be firm and consistent with what categories you sort things into. If you want this sword to work for existing dnd monster types or even alignment then I guess that works too..... I think if I were going to modify it any I'd just use this same concept but affecting what language if any the targets spoke.
............god DAMN it that's such a better iDEA i made this whole GRAPHic mother FUCKer....