I actually think Gerald Ford in '84 is a possibility too. As already recapped, Ford remained fairly popular during Carter's term, was considered the co-frontrunner for the 1980 nomination with Reagan, and was still the erstwhile leader of moderate Republicans. Had Reagan lost in 1980, I wouldn't rule out Ford being a candidate in 1984. There'd be a backlash to the conservative wing of the party and Ford might still be the most prominent non-Reaganite. I don't think George Bush, Sr. would be that compelling a candidate, having been Reagan's (losing) VP candidate and having never been elected to anything other than a Texas congressional seat.