I see a huge opportunity to advance the sub system through performance based and participation based pay scaling. The sub system offers a very attractive position for several different types of people; those who are right out of college and searching for the next opportunity, or people trying out teaching, supplementing income from another part-time or personal endevuer, retired teachers or other retired folks. All these people have very different needs and aspirations in the sub system. Most are very temporary(ask sylvia about these stats), some are full time "career" subs, some are part time regular subs(with a definite schedule), and some are as needed, special assignment type subs(Lynn). Some do long term, some do short term.
First year subs are by nature less valuable than a veteran sub who is familiar with the schools and the teachers and kids at the schools. This is the most obvious implication of performance/experience based pay scale; an experienced sub can simply get more accomplished and be more effective in the classroom.
This example of different classes of substitutes is only one metric of the pay scale idea. With a web system, it is possible to collect teacher/student, substitute, and administration generated social metric inputs that can be applied to a flexible pay scale.