The Kirby heirs should get better counsel maybe. Anyway, a fact remains: Kirby kept using the same concepts throughout his career and it is very easy to unearth those and Marvel won't be able to sue for plagiarism. Indeed that's exactly what's happening with Kirby Genesis at IDW. They have an Aesir called Sigurd (very much like Thor), a character called Swan who is a type of Valkyrie, etc.
Of course, it still requires top creators like Busiek and Ross to flesh the characters and make them interesting.

However, without the Lee touch, many of the Kirby concepts never went far. But the Lee touch isn't trademarked and nothing prevents current writers from bringing it to creator owned characters like Fighting American. (General Glory in the JLI is an example of how one could do Fighting American)