As far as who is going to be the 4th receiving option (TE or WR), I think it boils down to who they like more on the field for the majority of snaps (Dreesen or one of our receivers not named Thomas or Decker). Right now I would expect Dreesen to be that guy.
I also don't see McGahee being a huge factor in the passing game. I think he's going to get relegated back to short yardage situations like he was in Baltimore. Both for the sake of reducing the wear and tear at his age and the fact that he's never been much of a receiving back.
Of course this is all before injuries start being a factor...