In general, damage-dealing spells do not seem worth upcasting; the damage just doesn't scale very well. This is probably by design, because no one would bother casting higher level spells if an upcast lower one was better. Regardless, I'd like to homebrew a magic item that increases the damage output of spell upcasting to be roughly the same as using an equivalent higher level spell. This is hard for me to figure out, however, because there are so many qualitative differences between spells - it's hard to sort out how much I need to buff in order to quantitatively make things balance out. I know upcasting damage spells usually isn't effective, but I don't know how much I need to add to make it worth it.
The Objective is to create a magic item that increases the power of upcasting to be roughly the same as using a higher level spell. An example might be "when you upcast a spell, add an additional damage die to all damage" - except that seems like too much. I cannot provide a specific example of a spell I'd like to make it easier to upcast because the idea here is to create an item that applies to many spells. I am open to making the item have a limited number of daily charges.