I have mixed feelings about having a separate "Illusionist" class. I've been playing D&D for over 25 years, so I've seen a lot of ideas come and go. If I recall correctly the original "Illusionist" was introduced as a separate class in the first "Unearthed Arcana" supplement. I was intrigued by the concept and rolled one up. Over time that illusionist became my most powerful character. And he was loads of fun to play.
When 3e came out, they rolled "Illusionist" back into the standard wizard class as a "specialist" and the result was a seriously nerfed character. The result was so poor from a game play perspective that I never did convert him to 3e, and never played an illusion specialist in 3e.
Part of me would be thrilled to see "Illusionist" again a separate class that is as powerful as other classes. I know that it is probably impossible to convert a 2e character to a 4e character, but I can still convert the concept and recreate the character as a 4e character if I want. Which would be fine with me.
But there are two serious concerns about Illusionists. One is about the class limitations, and the other is about the basic concept of illusion magic itself.
The first concern is that in 2e Illusionists were powerful party members and due in part to their unique abilities, they could perform multiple roles (e.g. my Illusionist was far more "sneaky" than any rogue). But they had a "glass ceiling" that cut them off at high levels because they did not have access to "Wish" or other ninth level spells (the highest level Illusionist spells in 2e were seventh level spells). If 4e is going to create a class around "Illusionist" I hope they do not restrict their highest-level abilities in the same manner. A high level Illusionist should have the same ability to create permanent powerful magic items as a wizard. Illusionists should not be "second class magic users."
The second concern around Illusionists is the nature of illusion magic itself. In 2e it was horrible. People talk about "save or die spells" as being something terribly broken in 3.5e, and I agree with them. But many of the Illusionists most powerful abilities were completely overcome if the DM decided to have his NPCs "disbelieve" the illusion. This works both ways, and in campaigns where NPCs used illusions, party members would enter every room or new encounter like this: "Looking for traps, listening, searching for secret doors, disbelieving everything I see." In 3.5e it got a little better, but illusions were easily defeated by a will save. The bottom line is that the Illusionist's greatest problem as a player character is that the DM KNOWS that the spells are illusions, and that affects how they play their NPCs. It is a rare and valuable DM who role-plays the NPCs appropriately when faced with illusion magic. In 4e if they do resurrect the Illusionist as a class, I hope they take a hard look at how to make illusion magic effective even in the face of DMs who unconsciously meta-game illusion magic.
If they do, I would love to play an Illusionist again. If not, I think I'll stick to wizards.