Even if your system were practicable – which I very much doubt – I
think it would not be a good idea for a very simple reason. If you
want the rating difference to reflect the expectation value of a
player's RESULTS in a tournament, you will have to base your system on
past RESULTS (like the current system), not on the strength of the
player's moves. There are important other factors that will contribute
to determining the outcome of his games. Take the extreme example of a
player who plays the best move in every position, but who just keeps
losing on time. In your system such a player would have a very high
rating, in spite of which one could confidently predict that in his
next tournament he would perform as lousy as ever.

Tobi