Vulnerabilities

A big beef I have is that monsters in the basic Monster Manual look a little bland after a while. Very few have an actual vulnerabilities. The vast majority are resistant to normal weapon damage, but not magical weapons. Eventually the GM has to hand out magic weapons to all the players or they start to grumble, then the monsters start to lose their flavour.

I'm fairly happy with the base damage types in 5th ed, I am always happy to add a few more Damage Types even the allignment based damage types can work. I am also happy for damage resistance to exist. I know it was removed in an attempt to stop PCs from becoming immune to horders of lower level monsters, but if used sparingly without PCs getting easy acces to it then it can be fun.

Some general vulnerabilities I'd like to restore:

Skeletal creatures have +4 AC versus piercing weapons, vulnerable to bludgeoning weapons

Fey creatures are vulnerable to Cold Iron

Undead are resistant to necrotic and poison, vulnerable to radiant and holy.

Wooden creatures like treants have resistance to piercing and bludgeoning weapons

Lycantropes are resistant to all damage apart from silver or magical weapons. They also regenerate rapidly unless the wound was caused by silver.

Jellies and Oozes are invulnerable to normal weapons, resistant to magical weapons.

Very fast creatures like quicklings have +2 AC versus weapons that are not light. Heavy or versatile or two handed weapons also have disadvantage against them.