I think the Bills are structurally sound on defense but do not have the exact right players to make it work. A few years back, Beane went all in on the defensive front. He already had Oliver and Epenesa drafted and continued with Rousseau, Basham, and then Von Miller. I don't prescribe to reaching for draft picks, or that they'll produce right away, but there is a way to reload the defensive line to make more sense.
When teams run a defense like Buffalo's, they either need a tremendous amount of talent in the starting 4, or they need to attack in waves. I don't think Beane will replicate Sapp, Rice, White, and McFarland. Or even Mario Williams, Kyle Williams, Dareus, and Hughes. But he can put schematically sound players in the right places.
Buffalo has tried that type of rotation, but I think they've went light on the DTs. I like players like Jordan Phillips, Jefferson, Johnson, etc. But they are journeyman rotation lineman. Not really the caliber of what is needed.
Rousseau: 4 down player. He needs to rush from LE. He comes off the field when he's tired.
Oliver: He's paid well but I think he's more of a pass down DT
Jones: run down 1T DT
Epenesa: run down RE
Solomon: pass down end, either comes in for Epenesa or spells Rousseau
Carter: pass down DT
Miller: I think unless he re-structures again, he's a post-June 1st cut.
With that, I like a rotation of:
Epenesa - Jones - xxx - Rousseau
xxx - Oliver - Carter - Rousseau/Solomon
To me, Beane should be in search of a pass down RE and a bigger, run down 3T DT. He'll also need a younger run down 1T DT.
Maybe a smaller UFA splash like Azeez Ojulari? There are some older DTs in free agency as well (BJ Hill, DJ Jones). I'd rather go with some younger kids at the position.
If I had to make draft picks right now, I'd take Tyleik Williams with the 1st rounder and double up with Dontay Corleone with one of the 2nds. They aren't reaches. Corleone reminds me a lot of Alim McNeill. Williams looks like a young Pat Williams to me. Which would make the DL look like this:
Run downs: Epenesa - Jones/Corleon - Williams - Rousseau
Pass downs: Ojulari - Oliver - Carter - Rousseau/Solomon
For sure, the Bills could use better OT depth, a better starting safety, and need to figure out what to do with Rasul Douglas and Amari Cooper. But this is where I'd continue with the rebuild.