I like the concept. Here are two things to consider.
1. This is all about democrats and republicans. What about all the progressives and libertarians? I don't think we can assume that either party would fairly represent interests other than their own. There's no way the current neo-lib democratic party would nominate a real progressive.
2. Consider term limits. Perhaps after 10-15 years, a Justice's term would be over and he/she could be renominated or retired.
Could this all be done with legislation or would there have to be an Amendment to the Constitutional?