First off, let me say that I am not posting this because I think it's a good idea - I'm posting more out of curiosity about what other Rumblers think. I'm also not advocating firing Wannstedt mid-season. I think it is too early to tell whether the failings of the Bills defense are being caused by a "vanilla", predictable scheme or by a failure to execute that scheme, whether due to a lack of effort or a learning curve.
Singletary is currently in his second year as linebacker coach for the Vikings, after having been fired as head coach of the 49ers in his second year, 2010. By all accounts, Singletary has a temper and a fiery approach and had mixed success with the 49ers. He is clearly responsible for some of San Francisco's current dominance, as Harbaugh made few personnel changes when he took over last year. The Vikings have clearly overperformed this season, both on offense and defense. Minnesota's defensive success is primarily attributed to their front four pass rush and not their linebackers. But this might be a positive, as Buffalo would also like to generate a consistent pass rush using just the D-Line. Singletary would have some familiarity with that approach.
Singletary is a HOF, 10-time Pro Bowl linebacker. The general consensus is that the weakest point of the Bills' defense is the linebacker corps - not hitting the right gaps, not covering well, etc. Who better to assess whether we have the personnel we need, and, if not, who we need to draft or acquire through trade or free agency to shore up the linebacking positions? Most of Dave Wannstedt's NFL success was in the 90's. Arguably the league has changed since then, particularly in terms of offensive play and how defenses evolve to match it.
Also, many on this blog have lamented the need for a coach with a some fire to motivate players and push them if they aren't producing. Singletary, also a minister and motivational speaker, seems like that guy.
On the other hand, Singletary's style is different than Gailey's and my guess is that the two wouldn't get along nearly as well as Gailey and Wannstedt. Maybe they wouldn't work together at all.
There are Rumblers who know much more than me about any of this. What do you think? Any other possible candidates?