So this is already available to you.
Expert RAW outputs RAW photos that have been processed, as a matter of fact, and are not unadulterated images like Pro mode RAW photos.
Is not the point of RAW to have better processing results in post? So if your Pro mode is dark, couldn't you just lighten up your image in post? BTW, I was reading one user's comments somewhere about how ExpertRAW actually creates output that has degraded potential for lightening up the shadows. The lighter output you claim to see coming from ExpertRAW might still be lacking detail in the shadows. What you see is the best you'll get while Pro RAW output can be pushed more to bring out details hidden in shadows.