So now that I'm off work and can apply a bit more mental bandwidth to the topic I would probably word it like follows (Disclaimer: I just wrote it up off the cuff and put no thought in fully balancing):
Wizard's Flair
Mages, almost to a fault, are notorious for their tinkering of their spells and researching new ways to bend the laws of reality to their will. While every caster will see their magic affected by their personality and moods wizards are the only casters that forge and guide these changes intentionally. This is usually apparent in the way that a spell's observable effects manifest. Maybe one wizard's magic missiles appear to be miniature Tarrasques flying at their target while their companion's spells are accompanied by the sound of celestial flutes and the next wizard decided that every spell with a visual effect will show the spell in a deep purple hue. No two wizards cast spells the same way if they can help it.
Starting at 1st level whenever you cast a spell you can alter how the spell appears to onlooker's senses. This could be a change to the color of the energy the spell presents (a firebolt is now silver instead of the normal flame), the sound a spell makes (A fireball moans instead of the explosive blast you would expect), the shape of the spell (A wall of force has visible religious symbols glowing on its surface) or anything else that the player and DM deem appropriate. These changes are purely cosmetic and do not affect the spell's game statistics at all. A player also cannot use these changes to amplify the spell such as making a minor illusion louder than a scream or making a light spell brighter than it normally is. These minor changes can be applied an unlimited amount of times.
Starting at 5th level the Wizard gains the ability to apply alterations to their spells on the fly with much more substantial effects. Choose a flair from the options below. When you finish a long rest, you may replace your flair with another. Whenever you cast a spell, you can choose to cast it with your flair. You can apply your flair to your spells a number of times equal to your Intelligence modifier. Applying it to one casting of a spell counts as one expended use, regardless of the amount of targets the spell or flair affects. You regain all expended uses of your flair when you finish a long rest.
At 5th level you can choose from the following pieces of flair:
Arcane Advance
Your flair causes the spell to batter the
opponent to the point where it's hard for
them to maintain their position. Choose one
creature affected by the spell. The creature
must succeed on a Strength saving throw or
be pushed 5 feet directly away from you. If the
creature cannot be pushed due being adjacent to
an obstacle the push effect does not go off.
Emanation
Your flair causes your spell to be cast with a
lingering Aura. Choose a creature within 30 feet
of you that can see your spell being cast. The
creature must succeed on a Wisdom saving throw
or emanate a faint aura. This aura prevents the
creature from hiding for 1 round.
Kindle
Your flair causes your spell leak energy from
another plane. Choose a creature within 30
feet of you that can see your spell being cast.
The creature must succeed on a Dexterity
saving throw or take 1d4 points of damage
of an energy type chosen from the following
list: Acid, Cold, Fire, Lightning, or Thunder
Slow
Your flair causes your spell to be cast in a
way that thickens the air. Choose a creature
within 30 feet of you that can see your spell
being cast. The creature must succeed on a
Constitution saving throw or have all methods
of movement slowed by 10 feet for 1 round.
At 9th level you can added these pieces of flair to the list of options to choose from:
Awe and Wonder
Your flair makes your spells flashy and
magnificent. They might have harmless
sparks, auditory effects, or beautiful displays.
Choose a creature within 30 feet of you that
can see your spell being cast. The creature
must succeed on a Charisma saving throw or
be charmed by you for 1 round. If the
charmed creature takes damage from you or
an ally while it is charmed, the effect ends.
Big Boom
Your flair causes your spell to be cast with a
thunderous boom. Choose a creature within
30 feet of you that can hear your spell being
cast. The creature must succeed on a
Constitution saving throw or be deafened for
1 round.
Distracting
Your flair causes your spell to be distracting,
or even enticing. Choose a creature within 30
feet of you that can see your spell being cast.
The creature must succeed on a Charisma
saving throw or make its next attack roll with
disadvantage.
Flash of Light
Your flair causes your spell to be cast with a
brilliant flash of light. Choose a creature
within 30 feet of you that can see your spell
being cast. The creature must succeed on a
Constitution saving throw or be blinded for 1
round.
Pungent Smell
Your flair causes you to emit a pungent smell
whenever you cast a spell. Choose a creature
within 30 feet of you as you cast your spell.
The creature must succeed on a Constitution
saving throw or be poisoned for 1 round. A
creature poisoned in this way will not
willingly move closer to you.