So, since I started working with this story idea, I've got two ways this could go, and I would like your opinions: one way is that Gibbs becomes a Master Sergeant and then a Master Gunnery Sergeant, eventually serving as a senior weapons instructor at Scout Sniper School or the lead sniper for Marine Forces Atlantic/Pacific (which is about how high an enlisted Marine Corps sniper can go, according to my complete lack of military expertise and Wikipedia/other online sources), which is where he'd be when NCIS comes investigating him for the murder of Pedro Hernandez.
The other way is to have Gibbs become a First Sergeant and then a Sergeant Major, which would put him more in a leadership role than as a sniper. In this version, I see Gibbs concluding his career as the Sergeant Major of the entire Marine Corps Reserve and stationed in New Orleans (where he would become friends with NCIS Special Agent Dwayne Pride) - which is where he'd be when NCIS comes investigating him for the murder of Pedro Hernandez.
Meanwhile, away from Gibbs and the Corps, his absence from NCIS would affect the rest of the cast in various ways - for instance, Tony DiNozzo, Kate Todd, and Ziva David wouldn't join the agency either (although Ziva may or may not've served as Mossad Liaison Officer if Jenny Shepard can still become Director of NCIS without having worked with Gibbs); Abby Sciuto might not be as spoiled as she is by Gibbs and probably won't be working by herself or allowed to so blatantly violate federal office dress code - or she might not be at NCIS Headquarters, but at one of the other NCIS field offices or working for a different agency; Ari Haswari would still be alive and likely either still working as a double agent for Mossad and Hamas, or possibly climbing the ranks and potentially overthrowing his father as the Director of Mossad; and more.
Anyways, any input would be welcome.