I bought this game as bargarian like 2 years ago. Good game too sad that seems Take 2 Interactive let the franchise die and no more sequel. I don't know maybe didn't sell well on it's time
EDIT It seems this aclares the question
http://flyingflip.com/index.php/faq
***Q. Is the Myth series open source?
A. No. The Myth III source code is owned by Take Two Interactive Games. The series was obtained by Take Two when Bungie was purchased by Microsoft. As part of the stock-buyout, Take Two received the rights to the source code and the name Myth. Neither FlyingFlip nor any other myth developing group owns all or any part of the Myth source code from any game.***
For what is said there when Bungie was bought by M$ sold Take2 retained the rights of the Myth series