Thanks for the suggestion Alexis. I think there could be technical and legal issues with providing users with a way to use map content from other sources. We've added the suggestion to include IGN maps to out Feature Requests list though (number 122), where people can vote for it.
I discovered brouter-web (https://brouter.de/brouter-web/) and I find the idea of adding our own maps very interesting.
I'm french and it would allow me to use plotaroute with a background map named IGN or even an overlay like the heat map of strava.
So if you find it interesting don't hesitate to implement it.