An illusionist needs, first of all, imagination.
A non-imaginative player will find it hard to play anillusionist, for saying: "I make the illusion of a scary monster" is not enough. He must work out the details. Take David Eddigs' Belgariad and Malloreon series: a demon summoner had to maintain a credibleshape for his demon with the details worked out, lest he would lose control of it... now, summoning is a different matter, yet the maxim stays: you must convince reality, your target, and, in case of a summoning, also the creature you call.
The GM couldassign a 'reality' bonus for a good description.
Kinslayer did a great job at describing the difference between an outside illusion you see, and the one in your mind - I'd classify only the first kind as an illusion, the others are hallucinations
and belong into the realm of mind control.
So, a wizard would place strands of magic and let them shape light and sound to create an image. That image is seen.
If he is also capable of weaving force, (telekinesis and the like) he can let a field of strength accompany the image, granting it substance and the ability to actually do harm or move objects. If he can also use mind magic, he could place a suggestion on it, making it seem more real, or grant it a semblance of independence, a program so that he does not have to steer it all the time. Thus, he could create a magical creature thatlasts as long as his power does.
As for true illusionary beasts - an insubstantial creature with magical ability could make a fake shell to allow mortals to interact with it easier. This could be the way how ghosts are percieved - they could have an innate ability to project an illusion, so you could judge a ghost's wits by how elaborate and realistic his image is. Now, a ghost appearing as a symbol could either be totally stupid, or see no point in appearances, and go for simplicity
As for illusionists, I see two kinds:entertainers, whoare open about using illusions, and those who hide it totally, not admitting to use even one spell that's 'not real', and use illusion to gain an advantage.
Illusions anchored to a substantial skeleton could be more durable and realistic - one meant to disguise a person would be more real than one feigning the existence of such a person, and an illusion of skin covering a skeleton warrior would need no force effect to break a skull. A golem might also be made more pleasant to look at through an illusionary skin.
As for power and cost - a simple illusion can be erected through basic knowledge of light and the like, with little energy expenditure, but the mana cost and necessary spells to be know increase exponentially if it is mean to exert force, and be independent (pinnacle of the illusionary's quest to make his dreams real). So the grading could be: Illusion, tactible illusion (minion), creation (semi-real), independent creation (a semblance of sentience) and permanent independent creation (a being of magic).