Arcane Spark

With a snap and a word lightning sparks against a creature or target within range. Make a ranged attack roll against the target. On a hit the target takes 1d6 lightning damage.

If you roll a 6 on a d6 for this spell, you can roll another d6, and add it too the damage. When you cast this spell , the maximum number of these d6s you can add to the spell's damage equals your spellcasting ability modifier. For each additional dice you can choose to make an Arcana check, a success lets you determine the damage type of the dice. The check is harder for each additional type of damage you try to add. 

Cantrip Upgrade. The damage increases by 1d6 when you reach levels 5 (2d6), 11 (3d6), and 17 (4d6)