Thinking lately about weapons imbued with magic. And about how 5E handles resistance, immunity, and vulnerability. I had the idea of adding a damage die. Mostly because I feel a sword should still do its damage type as well as magic. Rather than all of its damage as a magical type.
For example, a flaming sword. Rather than doing all fire damage. The damage roll adds 1d4 of fire damage. Any resistance, immunity, or vulnerabilities to fire applies to only the d4 and not the sword damage die.
The to hit bonus would determine how powerful the weapon. Which would determine the die size of magical damage. +1 would equal a d4, +2 a d6, and +3 to a d8. Anything greater is religated to greater monster abilities and/or Arifacts.