What's the worst that could happen?

True Strike

You extend your hand and point a finger at a target in range. Your magic grants you a brief insight into the target's defenses. On your next turn you gain advantage on your first attack roll against the target provided that this spell hasn't ended.

Literally the only way to make the worst spell worse

"That guy 👉 is holding a shield"

Creation:

You pull wisps of shadow material from the Shadowfell to create a nonliving object of vegetable matter within range: soft goods, rope, wood, or something similar. You can also use this spell to create mineral objects such as stone, crystal, or metal. The object created must be no larger than a 5-foot cube, and the object must be of a form and material that you have seen before.

"How far away is the BBEG's fortress?"

"His lair is still many leagues in the distance. You can just barely see it on the horizon, nestled in the-"

"I spawn a really big boulder on top of it. Like, a reeeally big boulder."

"..."

Removing the radius definition from literally any spell

Power Word Kill

You utter a word of power that can compel one creature you can see within range to die instantly.

No more 100 hp limit. Instant death for anyone.

My favorite magic item I ever got was Ring of the Grammarian. Change one letter of a spell...

Me: "Incestation" Dm: "hello, step brother."

Remove the risk of losing the ability to cast Wish.

Remove the limitation on what you can do with the second action from Haste.

A lot of spells only have one sentence about resisting or avoiding the effect. Like remove the save from Polymorph on an unwilling target, or remove the ability to discern most illusion spells.

Hold person: At the end of each of its turns, the target can make another Wisdom saving throw

You could start reviving unwilling souls (theurgy school).

You could also remove the HP requirement from power word kill to kill any creature instantly.

Disintegrate without having to fail the dex save is also pretty funny.

Light: You touch one object that is no larger than 10 feet in any dimension. Until the spell ends, the object sheds bright light in a 20-foot radius and dim light for an additional 20 feet. The light can be colored as you like. Completely covering the object with something opaque blocks the light. The spell ends if you cast it again or dismiss it as an action.

I just like the idea of giving each wizard a light pill.

You could instead strike out the size limit to make the world glow.

Control Flames

You choose nonmagical flame that you can see within range and that fits within a 5-foot cube. You affect it in one of the following ways:

You instantaneously extinguish the flames within the cube

I turn off the sun

" Create Water" cantrip.

Ignore the limit of 2 gallons per level.

The cleric/druid can now summon oceans worth of water, at will.

Combine with the "Shape Water" cantrip, and your wizard and cleric are now an unstoppable force at level 1. Especially if you ignore the limit of abilities the wizard can manage at a time.

Remove the rule where Shadow Conjuration requires the spell you mimic to be Conjuration (Summoning) or (Creation). Alternatively, remove the rule that gives this spell a level cap.

Remove the time/duration limit from Modify Memory.

Remove the "summoned monsters can't summon monsters" rule from your Summon Monster spell of choice.

Remove the "you must have a piece of the subject creature" rule from Ice Assassin.

Remove "Saving throw: Will negates" from Charm Person.

Remove the spell slot requirement

Remove the targeting requirements of power word pain . pick a random person. Start each morning by casting power word pain on them, no matter where they are in the world or what theyre doing.