Magic Item Resistance in 5e
So as I've been playing, I have noticed that it is almost impossible to not hit a creature in D&D 5e, no matter what class or level you are. I understand that this is the point of the system's Bounded Accuracy where even lower level characters and wizards can hit a higher-level monster, and uses Hit Points as a scaling tool instead. I have always loved this idea because it makes it feel like you can beat anything if you are clever or careful enough.
However, in the case of much more powerful creatures, I think it might be useful to give them some sort of resistance against low-level characters. This would make it easier for a DM to keep a creature alive for plot reasons and allows players to feel advancement as they level up. So here is a little tweak I made which you can use as you wish.
Certain monsters already require magic or magic items to fight it and they just have resistance to non-magic weapons. For a high-damage barbarian, fighter, monk, ranger, or rogue, this is essentially an annoyance rather than a deterrent. Such a creature is meant to be difficult or tricky to fight without magic weapons, but instead only sets the player(s) back a few rounds.
Instead, give monsters immunity to weapons below a certain rarity. Save this for monsters that you might want players to run away from, or else creatures you might feel have some innate magical resistance, like incorporeal undead or powerful monstrosities. I might even suggest giving ones with rarer item requirements resistance to magic items and immunity to nonmagic ones. Something like this:
Immunities: bludgeoning, piercing, and slashing damage from nonmagical attacks.
Resistances: damage from magic items or magic weapons of Common and Uncommon quality.
Immunities: bludgeoning, piercing, and slashing damage from nonmagical attacks.
Resistances: damage from magic items or magic weapons of Common, Uncommon, and Rare quality.
Immunities: bludgeoning, piercing, and slashing damage from nonmagical attacks.
Resistances: damage from magic items or magic weapons of Common, Uncommon, Rare, and Very Rare quality.
Immunities: bludgeoning, piercing, and slashing damage from nonmagical attacks.
The idea is reminiscent of 3.5e's Damage Reduction, where only certain weapons could harm a creature unless the attacker hit very, very hard (often needing to deal over 10 or over 15 just to deal more than 1 damage!)
This is just a rough idea and I definitely wouldn't use this for every single creature unless your campaign is just riddled with magic items.