Personally, I think we should have at the moment 104 Senators, with the 101st to 104th Senators being Senators Carter, Bush, Clinton, and Bush Jr. by virtue of being ex-Presidents. (Presidents who resign or are impeached would not get that privilege.)
Assuming they would be granted that position for the rest of their lives, I'm not comfortable with the idea of legislative officeholders not having to face the electorate on a regular basis. The idea of GWB being rewarded for his piss-poor presidency with a lifetime role in a different branch of government is not a savory one. Senator Carter would have been a member of the Senate for the past thirty years, and not had to face an election once.
