A player of mine has a cool idea of a dragonborn using his breath weapon on his sword to get an elementally-charged sword that would deal extra damage. I like the concept, discussed the general lines of the power with the player and came up with this first draft:
As a bonus action, you can consume a use of your breath weapon to imbue with elemental energy a melee weapon you are holding. On a hit with the charged weapon, the attack deals the damage of your breath weapon in addition to the normal damage. The weapon loses this charge on the first hit, if you are no longer holding it or after a number of rounds equal to your Constitution modifier (minimum 0, meaning it ends at the end of the turn you activate it).
For PHB dragonborn, this could replace the breath weapon but since Dragonborn are not the most powerful of races I'm tempted to just give it to the character for free. How balanced would this be with other standard races?
The question arose before the new dragonborn races from Fizban's Treasury of Dragons came out. I might end up using those instead but they do not need an extra boost. In that case this ability would replace the second breath/emenental immunity/flight that they get.
The player in question intends to play a Fighter (Samurai) with "maybe a bit of sorcerer". Problems with other classes (like Paladin or Rogue) are less of a concern for me here but still worth being brought up.
Tweak suggestions are also welcome.