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)
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 must make a Constitution saving throw or take 1d8 poison damage.
At 5th level, the melee attack deals an extra 1d8 poison damage to the target, and the damage the target takes if it fails its saving throw increases to 2d8. Both damage rolls increase by 1d8 at 11th level and 17th level.