I always define a HOF by how dominant they were during their playing years. Zach either led the league in tackles or was near the top almost every year as a Fin.
He was the heart of the team, and a presence that other teams planned to avoid. Give me two other LB's during his era that will make the HOF (Ray Lewis, has the rings) and Brian Urlacher.
Urlacher will make the hall because he played in a bigger market, period. Zach is just as good if not better that Urlacher.
So in short: Should he get in? Yes. But will he get in? No.