Not to infringe on your idea but I think it could also be cool if we just played our heroes through the ages but not as immortals but as each successor of the mantle.

We start whenever we want to start and play that character until they grow old and pass on the title to their chosen heir, or maybe they get killed and somebody honors them by becoming the  next generation. It would provide a lot of freedom with characters dying and role play styles being able to change from generation to generation. Also super easy for people to come and go without difficulty or complication for anyone involved.