Just an idea that popped into my head but I didn't have the time to write down.
Jack, Buffy and Methos were all Goa'uld hosts during Ra's occupation of earth
As they were all immortal, they could survive the removal of their symbiotes
Buffy and Jack are both at least as old as Methos, although one of them could have been taken as a host at the beginning of Ra's occupation
Buffy being famed as a particularly evil/brutal/psychotic Goa'uld
Buffy and Methos find out about the Stargate being operational again and go to Cheyenne Mountain...