Based on how Wikipedia describes derivative:

“In copyright law, a derivative work is an expressive creation that includes major copyright-protected elements of an original, previously created first work (the underlying work). The derivative work becomes a second, separate work independent in form from the first. The transformation, modification or adaptation of the work must be substantial and bear its author’s personality to be original and thus protected by copyright. Translations, cinematic adaptations and musical arrangements are common types of derivative works.

Most countries’ legal systems seek to protect both original and derivative works. They grant authors the right to impede or otherwise control their integrity and the author’s commercial interests. Derivative works and their authors benefit in turn from the full protection of copyright without prejudicing the rights of the original work’s author.”

This is a very grey area to uphold in legal terms. Many articles state that derivative claims fall to the judgement of the court. Although not a mandatory option in the law, people willing to do derivative work, should ask the originator. However, as the world is, many people turn a blind eye on what is morally correct and tend to steal good ideas to benefit their own game.