• "May" is used to express a wish or hope for something in the future.

• In the sentence "May you live long!", the speaker is expressing a desire for the listener to have a long life.
• "Might" suggests possibility, "should" suggests obligation, and "ought to" is similar to "should" but weaker. These modals wouldn't convey the well-wishing sentiment as effectively as "may."

Therefore, "May you live long!" is the grammatically correct and natural way to express this wish.