I feel like the only thing getting Dungy in is he was the first black HC to win a Super Bowl. He could never assemble a legit offense in TB, and he piggybacked on Peyton Mainning in Indy.
Winning a Super Bowl puts him in the HOF conversation, but he's a marginal candidate at best. Certainly not worthy of getting in ahead of, say, Jimmy Johnson or Mike Holmgren.
Being the first black super bowl winning HC isn't really part of his resume. It's part of the "league resume.". It's notable, but there are other ways to celebrate that fact in the hall w/o shoehorning Dungy in individually.