I would take one truly great interior defensive lineman. This scheme is just like the Patriots in that it enables individual pieces to look far superior to their true level. That's why so many New England defenders didn't look like much in their prior stop, nor their subsequent stop. It's also the reason I have to grit my teeth and mostly chuckle when posters here try to make Raekwon Davis out to be a world beater. Totally oblivious to situational influence.
If we had a truly elite interior disruptor then no chance that Denver game evolves the way it did. Heck, they wouldn't even attempt that game plan. Can you imagine intentionally using one slow developing parallel running play just behind the line of scrimmage, when you've got an Aaron Donald type, or even someone a cut below that, on the opposing defense? No chance. Those plays would be blown up and fans would be screaming at the absurdity.
Otherwise I agree that secondary is the least of the problems. Linebacker they need a 3-4 interior type with some bulk, mobility and instincts. For whatever reason we're struggling to identify that type right now, just like so long inability to locate offensive linemen. If I had to guess, Flores based on New England had the impression that Roberts was superior to his actual level.