Lions fans have probably skipped over this part and are ready to comment that Jim Schwartz is assured of his job in 2013. I understand that, but this is for 2014 and assuming that Reid takes a year off from coaching.
If the Lions underachieve in 2013, then expect Schwartz gone. Reid would be a great replacement for him. He'd work well with Matthew Stafford and Calvin Johnson, while adding much-needed discipline to the team.
The offense doesn't need much work—Reid's impact would be on the disciplinary side. Gone would be the days of the dumb penalties and shenanigans which have cost this team in the last two years.
Again, this is more of a 2014 suggestion, in much the same way that the New York Jets, Dallas Cowboys and Miami Dolphins are 2014 suggestions. For Reid, taking the season off might be the best thing for him and possibly open up better opportunities down the road. The Lions represent this.