2 big issues are missed assignments and penalties. Those are coaching issues. The OC or OL coach needs to be replaced, or need to rethink how they are approaching things because it's not working. The O-line wasn't perfect last year, but they didn't suck like they do now.
If nothing changes, this is the best we can hope for:
Leno and Massie return to 2018 form
Whitehair continues to play well.
Daniels develops to be a good LG.
Bars or Coward develops to be an NFL caliber RG.
The holes that need to get filled are RT, LT, and RG. A tackle will probably take a high draft pick or some FA dollar, while a guard could be found in later rounds in the draft or cheaper than a tackle in the FA market. Massie and Leno's contracts make them hard to cut, but they certainly can use some competition. They would be upgrades over the backups we currently have, so we could keep them on the roster. We definitely need a TE, and I think much of the OLs woes have been impacted by the poor blocking of the TEs.
When you add it all up, there's 4 positions that need to be replaced by hit the ground running players, since this is the final year of Mitch's contract. Considering the lack of capital that we have in the draft, and limited space in free agency, I highly doubt that all 4 positions could be filled in the offseason effectively. I certainly hope so, but it would be close to miraculous. I think we're going to have to hope for some guys on our current roster to step it up in 2020.