What good essay topics are there for King Lear?

There are a ton. If your teacher talked at all about sight/blindness, then that will surely be one (exploring the use of imagery and knowledge). A question about duty - duty to parents, to friends, to rulers - is also possible. That is, discussing, the various breaches of duty and what pain they cause. Talking about wisdom vs. foolishness is another good possibility - consider the wise Lear's folly at the beginning vs. the wisdom of his fool.