Spoiler:After defeating the final boss, The Harbinger of Doom, he will exclaim to BJ Blazkowicz, "Your descendants will pay!"

This suggests that Doomguy (the hero of Doom) is a descendant of BJ Blazkowicz, the hero from the Wolfenstein series. B.J. Blazkowicz also has one confirmed descendant named Stan Blazkowicz, who is one of the three main characters in Doom II RPG.
Contributed by pAroxizm