I would rather see it as its own class. For one thing, I think that it should have non-vancian resource management. An illusionist seems best suited to having many weaker spells that can confuse, disrupt, and cause all sorts of mischief. That simply doesn't sit right in a vancian system, where spells are major important resources to be used with care. If the illusionist was merely a wizard specialization, then his illusion spells would have to be on the same power level as normal wizard spells, and that seems too strong for most illusion spells. An illusionist should, in my opinion, have a good list of at-will spells that expand as he levels, giving him great versatility. On top of this he should have either encounter abilities or daily abilities. If he has dailies, they should be like the 3e sorcerer. Encounter powers would best be managed as spell points.
Actually, if I were to design it, I would give the illusionist spell slots. Half the spell slots would be encounter, and half would be daily, but they would tap the same pool of spells. So if an illusionist had 6 spell slots, then the first 3 spells he casts in an encounter would recharge after the encounter. The slots he expends after this would come out of his daily pool. He would have a daily resource that would let him push himself, but each encounter he would always have about half of his potential ready to go no matter what.
That's just one way to do it though, and probably not a very good way. The point is, I want an illusionist to be activating and maintaining many illusions every single encounter, and that doesn't sit well with the wizards vancian mechanics.