Should all mages be able to turn invisible, mirror image, blur or conjure phantasmal force/minor images to trick opponents? Sure.
Should all mages be able to use Color Spray, Hypnotic Pattern, create a mental image of fear so terrible it might kill you, be able to weave light and shadow to the point of being semi-real? No. That's the Illusionist's shtick.
But it is still, at its essence, a full arcane caster class. There isn't any (or enough) distinction that, for me, warrants its entire own class.
Theme please.
Generally speaking, to my mind the Illusionist would be helpful/useful for the Exploration and Interaction pillars more than the Combat, as much as if not moreso than a "full/general" mage. But still can help out/defend themselves in combat/in a pinch/as necessary.
Illusionist -Theme (or Advanced Theme, however that's going to work)
-Forgoes the Mage spell list and chooses only from the Illusionist Spell List
-Illusionist cantrips: Detect Illusion, Dancing Lights, Ghost Sound
-Bonus to saves against Illusions and Mind-effecting magics, penalty to target saves against their illusions.
-Associated Bonus Skills: Sleight of Hand, Performer, Disguise.
Illusionist Spell List
level 1.................................level 2..............................level 3
Chromatic Orb...................Blur...................................Blink
Color Spray.......................Detect Invisible..................Clairvoy./aud.
Darkness/Light..................Hypnotic pattern................Illusory Script
Disguise Self......................Invisibility...........................Invisibility Sphere
Silent Image......................Magic Mouth......................Major Image
Sleep.................................Minor Image......................Nondetection
Ventriloquism.....................Mirror Image.....................Phantom Steed
Wall of Fog........................Obscure Object..................Suggestion
I'll leave higher levels to your imaginations. But you see the pattern/get the gist.
--SD