I am considering the following homebrew magic item for my campaign:
Ring, rare (requires attunement)
While attuned to this ring, when you cast a cantrip that targets a single creature (but does not have a range of Self), you may choose to expend a spell slot to have that cantrip target an additional number of creatures equal to the level of the spell slot. Any additional creatures targeted must be valid targets of the spell, and no creature may be targeted more than once.
If the cantrip requires an attack roll, make a separate attack and damage roll for each creature. If the cantrip requires a saving throw, each target makes a separate saving throw but takes damage based on a single damage roll.
I really like how this feels for low level parties. For example, a level 3 wizard using a 2nd-level spell slot to attack 3 creatures for 1d10 damage each with a Firebolt seems perfectly reasonable. I am NOT happy, however, with the idea of a level 17 wizard using a 9th-level spell slot to attack 10 creatures for 4d10 damage each.
How could this magic item be improved so that it is still interesting for low level characters without being so powerful at higher levels? Or am I overestimating its usefulness at high levels?