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 it becomes outlined in sparkling light until the end of your next turn. While outlined, it sheds dim light in a 5-foot radius and cannot benefit from being invisible.
At 5th level, the attack deals an extra 1d6 radiant damage to the target. The damage roll increases by 1d6 at 11th level and 17th level.