Nothing is impossible, and NFL truths have certainly proven to be stranger than fiction many times. You can take Parcells coming to Dallas and Gibbs returning to Washington, after categorically denying many times he would ever leave NASCAR, as exhibits 1A and 1B.
That said, I thought I'd read in a recap of an interview here several weeks ago where Johnson had emphatically said he loved his life too much to go back to the daily grind of coaching. Something about "it's fun several days a year (usually game day) but not fun the rest of the time".
Anyhow, here's what part of the staff may look like when he's here next year:
Asst. Head Coach/ OC: Norv Turner (promoted, thereby able to get out of SF)
Def. Coordinator: Butch Davis
Secondary coach: Dave Campo
O-line coach: Tony Wise
WR coach: Hubbard Alexander
ST coach: Joe Avezzano
: (although all these guys could be had, either due to being currently unemployed or being able to switch teams/get out of current contracts. Jimmy always did say he felt much more comfortable w/ his assistants around him).