You're right, the Falcons need to address the CB position this off-season. At the very least get a competent nickel back that can stay on the field (unlike Chris Cash), that can potentially push Webster if Williams isn't up to it.
But first and foremost, the Falcons defense needs to figure out what style they want to be: zone or man. They seem to be wanting to have it both ways. I realize that no team even the Bucs are exclusively one or the other. But continually over recent years, we've seen the Falcons talking about having top cover corners (which roughly translates to man coverers), but play a lot more zone coverage.
Basically the Falcons need to figure out which one they want to be, add the proper player, and stick to it during the season. Hall is one of those corners that could play effectively in both (although he seems a lot softer vs. the run than a normal Michigan CB). McCauley and Antoine Cason (if he comes out) seem more man-guys. Darrelle Revis could probably play both. Daymeion Hughes is a pure zone guy. They seem to be the consensus top 5, but as normal, somebody rated as a 2nd rounder now likely will jump up over the next 4 months.
As for RB, I think the Falcons need to bring in some size. All 3 of the top RBs (Lynch, Peterson, and Irons) would work well behind our zone-blocking running scheme. Peterson already plays behind one at Oklahoma. But are any the physical presence we need, and do we need to use a #1 pick there? Doubt it. I think the Falcons can address this area more likely in free agency and use the draft to build elsewhere.
Hey if Duckett doesn't get many sniffs, might as well try to bring him back cheaply.