There is no way to know what the other teams will look like (although Boise State's offense is set up to be scary good the next few years)...
So looking at Marshall... I would hope that the offense will improve. We only lose two seniors this year (assuming Obiallo can play 4 games and still get a 5th year redshirt)... The offensive line should be solid (4 starters return), I would assume Green will still be at Qb and hopefully improved with another offseason, and Knox/Evans are back as is Gaines at TE... What we need is Thompson, Gammage and Talik Keaton to make strides at wide receiver between now and next August.
Defense is another story... We lose a lot... The d-line only returns Hodge and Couch. The only linebacker returning is Beckett. In the secondary we return Johnson, Gilmore and Abraham... The defensive line should be okay with Croom and Burton back to take over starting gigs. Biggest need between now and next August is finding at least two starting caliber linebackers (preferably three, because you always need one ready to step in).
The good news is Yulee will be back next season at LB. Plus we have the units look like this for 2020:
QB - This unit has raw talent in Green, Wells, and Underwood but someone needs to step up and at least be an average FBS QB.
RB - Loaded. We have the best 1-2 combination among G5 schools and have good young RBs behind Knox and Evans as well.
WR - We only lose Tavin Richardson among those WRs that were contributors if Obialo and Henry both decide to stay for their SR years. Both had redshirts available and will meet the redshirt requirements.
TE - Big loss in Levias, but Gaines and Miller return and we may land a very good JUCO TE or two in the current class. The position should be solid in 2020.
OL - The OL should be returning every scholarship OL player except the dancing bear so the position group should be a major strength in 2020.
DE - Lose Couch but return Hodge, Cumberlander, and Burton and have plenty of young depth to the point that DE has not been a priority in the current recruiting class. Strong group overall.
DT - Lose Hames but return Croom and Edwards and have some young depth at the position. Looking to land a top JUCO DT in the class with the lack of experience among the backups. Should be solid position group in 2020.
LB - Return 5 scholarship LBs including Beckett and Yulee. The group needs depth so landing a JUCO LB or two is a priority in the current recruiting class. Have a solid 4 returnees with experience, but depth could be an issue if injuries occur.
CB - Lose Merrell and Jackson, but return Gilmore, McClain-Sapp and Abraham could move to corner. Also add USC transfer Pie Young to the group.
S - The group does not lose any players and top safety Derek Pitts will be available after redshirting in 2019.
PK - We lose Rohrwasser, but Ciucci is an accurate kicker and should be a solid replacement.
PT - Lefevre returns for his SR year so the position should be good in 2020.
LS - Lose Beardall after 50+ starts. Brodie Williams (R-FR) was a highly rated LS coming out of HS and should continue the long run of good long snappers.