Free upcasting items. How balanced would they be?

[Anymage]

I was thinking about the first form to make a caster who's especially good with a specific spell, and came up with the idea for the first item. Wondering if there's much useful concept space for either of the following:

Staff of Improved Fireballs(requires attunement by a caster who has Fireball on their class spell list)
You may cast the Fireball spell by using a spell slot of second level or higher, even if you don't know the spell or have it prepared for the day. When casting Fireball in this way, treat it as if it were cast from a spell slot one level higher than the one you spend.

Staff of Improved Upcasting(requires attunement by a spellcaster)
When casting a spell using a spell slot higher than its original level, treat that spell slot as one level higher.

[BerzerkerUnit]

Fireballs staff is probably broken, it one shots most T1 threats. It also isn’t in theme with intention. It makes the character a specialist rather than rewarding a specialist.

How about:
When you cast Fireball with a level 4 slot or higher you may reroll a number of damage dice up to the level of the slot. You must keep the new roll.

For improved ipcasting I might increase save DC by 1 for every two levels over (min 1)

So casting Fireball with a 4-5 slot is +1 dc a 7 slot would be +2. I might do the inverse with crit range for attack spells.

[sandmote]

I would add a free benefit for a particular spell, rather than tying it to upcasting. For example, +1d6 damage for a fireball by itself, instead of also letting the player cast it three more times without that bonus.

You could also give +1 to attack rolls/save DC for spells from a particular school of magic or type of spell, which is probably the simplest bonus. For an illusionist, you could also increase the DC of investigation checks made against their Spell Save DC.

If you insist on bonuses for upcasting, it might fit better to have an additional bonus the player can choose instead of the usual benefit from upcasting. Increasing the area of the spell instead of the damage, for instance.

If you have a particular group you can describe, it might be easier to give more specific suggestions.