I am considering the following homemade magic article for my campaign:
Rare magic ring, requires tuning
While in sync with this ring, when casting a cantrip that points to a single creature (but not to yourself), you can choose to spend a spell slot so that that cantrip targets an additional number of creatures equal to the level of the spell slot. Any additional target creature must be a valid target of the spell, and no creature can be targeted more than once.
If the cantrip requires an attack roll, make an attack and damage roll separately for each creature. If the cantrip requires a save roll, each target makes a separate save but receives damage based on a single damage roll.
I really like how this feels for low level parties. For example, a level 3 wizard that uses a second level spell slot to attack 3 creatures with 1d10 damage each with a Firebolt seems perfectly reasonable. However, I am NOT happy with the idea of a level 17 wizard that uses a level 9 spell slot to attack 10 creatures with 4d10 damage each.
How could this magical item be improved so that it remains interesting for low-level characters without being so powerful at higher levels? Or am I overestimating its usefulness at high levels?