Trademarks are designed to be forever, as long as they remain well known enough, but that doesn't prevent someone from using the concept instead of the actual i

Trademarks are designed to be forever, as long as they remain well known enough, but that doesn't prevent someone from using the concept instead of the actual image itself. i.e. You could write a book with the character mentioned in it just like you can write a book with Coke mentioned in it.