Offhands have never been a main hand thing, or even a direct weapon for combat, same as a shield.
Let’s say offhands do get illusion. While I would see it fun and not harmful at all, could feel abit off, because if offhands could have an illusion, why not shields? Why not bearforms? Just to say some.
Enchant illusions should apply to those weapons which are necessary to perform key abilities and/or 1h right hand weapons from a class, imo; such as… cough hunter ranged weapons. (Illusions and hunters).