I'm assuming you watch football on Sundays a decent amount. How many new plays do you really see? I think you could give the other coach the playsheet (with the names changed) but the actual plays and it wouldn't change who wins or loses. I can't remember the last time I saw a new play I hadn't seen before. There are some calls that come when you don't expect them, say an onside kick to start a half in the Super Bowl, but that's not the play, it's the combination of executing the play properly and the coach having the balls to call the right play. Some of it is just luck too. You happen to call a play that works against the blitz the defense happened to call on that play.
Let's not forget Gruden took a last place Raiders team and got them to .500 before he lost to the eventual Super Bowl Champion Ravens (minus his starting QB most of the game) and then lost to a pretty good Patriots team by 3 in OT the next year in the Tuck Rule game.
He did inherit a good team from Dungy, but Dungy had problems on offense. The three playoff years before Gruden took over, Dungy's TB teams scored 6,9, and 11 points. Ouch!