I think in terms of appearance versus reality, there are a number of illusions being used.
First, Lear's two daughters, Regan and Goneril, fool Lear into thinking that they are his most loving and trusted daughters when, in fact, they are going to strip him of his power and belittle him.
Likewise, Cordelia, with her silence, appears to be insolent and disrespectful. However, in reality she is his most loving and loyal daughter.
This is reflected in Gloucester and his sons.
Edmund tricks his father into thinking that he is really his most trusted son. However, that is far from the truth and he treats hif father much the same way Regan and Goneril treat Lear.
Edgar, on the other hand, appears to be the bad son and is banished under the threat of death, yet in reality, he proves himself to be as loyal to his father and Cordelia is to hers.
Now, the concept of illusion can be carried one step further when you think of the disguises that Edgar and Kent adopt in order to stay in Lear's kingdom to help him.
You could also explore whether Lear's madness is really an illusion or not, for it seems that in the beginning of the play when he appeared normal and had all of his power, he was acting very foolish. Now compare that to when he is kicked out by his daughters and left to wander the hearth, where he seems to be insane, yet he begins to regain some of his common sense and royal nature.