Instead of your double indicator, I would rather think of a very sensible "LOC trend" or "glide trend", as is done with the speed trend, displayed on the side of the diamond.
Then you would just set the LOC trend to zero, and if there was a deviation, you could see much faster if it is growing or reducing, which would also help the PM's job.
This is what the FD does. A raw data is raw data. You are suggesting a emergency or limited function FD. The technology may provide you an emergency FD using different inputs. As it is A350 gives you AP even with double engine flameout. It does auto TCAS, auto TCAP(TCAS prevention), auto EMER DES. They aren't going to develop anything to help you fly raw data. They will develop emergency AP. You don't need to practice everything raw. How many times FD are lost during RNAV departure and you have no alternative? A direct to or radar vector is always possible. As long as you can climb, descent execute an approach raw data within acceptable accuracy it is good enough.