I would imagine you have read them, but for others possibly interested in the story, there are two interesting books on the subject: The Saint and the Devil (The story of Joan of Arc and Gilles deRais) by Frances Winwar (1948), who looks to have done a great deal of research, and the much lighter, more contemporary Gilles and Jeanne by Michel Tournier (1983). If you haven't read the former, it has an extensive bibliography that might be helpful to you.
So for me, yes, I would think that a very interesting topic (and if he is indeed guilty of the murders, or whether he met his end for his previous staunch support of d'Arc). It would be wonderful to have an adventure game including Joan, where reflex action isn't necessary, and the complexities of the characters were revealed (no small task, I am sure!).
"How could drops of water know themselves to be a river? Yet the river flows on."
- Antoine de Saint-Exupéry