Brennan still has two (potentially one) remaining vacanc(y/ies) on his coaching staff, and they are big ones. With quarterback Noah Fifita and wide receiver Tetairoa McMillan announcing a return for 2024, among a plethora of other projected offensive starters, this is a position that's important to get right. With the right hire, Brennan could fill both vacancies with the same person. After all, it's not something he's shied away from so far.
My opinion? Give former New Jersey Generals (USFL) head coach Mike Riley a call. Riley was the Oregon State head coach from 2003-14 who hired Brennan onto his staff in 2011. He has both play-calling and head coaching experience, but he has shown in recent years that he isn't afraid to take a coordinating position. With the merger between the USFL and the XFL and the subsequent dissolution of the Generals, Riley is suddenly out of a job and could be looking for more stability in the college ranks. He was also the head coach of the now-defunct Alliance of American Football's San Antonio Commanders before that league also folded, partly due to COVID-19-induced financial strains.