I have created some new cantrips which include making a weapon attack as there are only two official ones to choose from. I've tried to follow these principles based on Booming Blade and Green-Flame Blade:
- Melee weapon attack only, 5 foot range
- No unavoidable extra damage to the primary target at level 1
- Damage no more than 2 targets
- Spellcasting ability modifier doesn't have to affect the spell at all
- Only available on the Warlock, Sorcerer and Wizard spell lists
But with such a small amount of official content to compare them against, it's hard to say if they're balanced.
Is this spell balanced with Booming Blade and Green-Flame Blade?
Casting Time: 1 action
Range: 5 feet
Components: V, M (a weapon)
Duration: 1 round
As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell's range, otherwise the spell fails.
On a hit, the target suffers the attack's normal effects, and you become charged until the end of your next turn. While charged, you deal an extra 1d8 lightning damage when making a melee attack with a weapon. If you become charged while you are already charged, the charged damage increases by 1d8, up to a maximum of 2d8.
At 5th level, the attack deals an extra 1d8 lightning damage to the target and the maximum charged damage increases by 1d8. The damage roll and maximum charged damage increase by 1d8 at 11th level and 17th level.