I think what we really need is one more very good defensive lineman. We have one in Ratliff, but Spears is just okay and Olshansky will just be solid, too. I think we need one more guy like Ratliff that can create havoc, whether that's a big, strong NT that can push the center into the backfield, or another disruptive end similar to Rat if he stays at NT.
We also need a good 3rd down rusher IMO. Ellis is about done and I hate his attitude. So we need to find a Shaun Phillips type to join Ware and Spencer on passing downs.
I also tend to think we need a really top, Pro Bowl type ILB next to Bradie. If Jay stays at NT, he needs to be a bigger, physical guy. If we got a big NT, maybe he could be a smaller, athletic guy that runs to the ball all over the field and makes a ton of tackles. This might not be as necessary as the other two, though. Or maybe it would be harder to find this than the 3rd down rusher -- I go back and forth on that.
The safety can be just a good player. Safeties don't make or break defenses IMO.