The what to do with an ex-President issue. Personally, I would favor amending the Constitution so that former elected Presidents would have a lifetime Senate seat once their Presidential term had ended. Might have made things interesting if both Clintons had been in the Senate.
But that has its own problems, since the Senate is a fairly small body and a few extra Senators could make a huge difference.
Make them take their state's junior senator's seat.
Although lifetime is a very bad idea, as viz. Ronald Reagan's retirement. Up to age 70 or something would do.