Senator Patrick Leahy (D-VT) has introduced a bill that provide that when an active Justice recuses himself or herself from a particular case, the Court's active Justices may, by majority vote, designate a retired Justice to participate in decision of that case. The current living retired Justices are Sandra Day O'Connor, David Souter and John Paul Stevens. Under current law, retired Supreme Court Justice may be designated to sit on cases heard in the Circuit Courts of Appeals but not in the Supreme Court.
As reported in an article in the Legal Times blog, there are concerns about how it would be decided by the Court which retired Justice would be chosen to sit on a particular case and skeptics of the idea say that litigant parties might attempt game the system of substitution to impact the outcome of the case.
Currently, 39 states allow retired Justices of their state Supreme Courts to be designated to sit on cases when an active Justice on the Court recuses. So far Leahy's proposal has not drawn much support.