Pace and Thomas are older than dust.
Scott is on his way out.
DeMario Davis is ascending and has not nearly met his ceiling. I have no doubts he will in a year or two.
Garrett McIntyre is OK. Not a superstar.
Harris is excellent, but that makes one.
Best OLB available, preferably one who can help to cover a TE or a slot WR.
I'd prefer one who spent his college days playing OLB so that its not another DE trying to learn the position when the NFL game is twice as fast.
If I had to choose another, I'd pick OG or OT. Howard/Slauson aren't the long term answer. Ducasse isn't very bright and is about to be inserted into a new system.