Denny Crane is a Pokémon.He has learned to say words other than his own name, but he lapses to being primal from time to time.
Crane, Pool, and Schmidt is a Branch of Wolfram and Hart.Come on, it would explain a great deal about why everyone is able to get away with so much. Why? Because they've literally made deals with the Devil to keep their jobs and get a good win ratio, even when it reshapes American law.
Denny Crane is the Devil himself.
The Ending Is a Sequel HookNothing's concluded in the series finale, and Alan and Denny seriously consider starting their own law firm. That's sequel bait.
The whole show takes place in Denny's headDenny Crane breaks the fourth wall a hell of a lot, most notably in "New Kids On The Block" where he welcomes new characters to Boston Legal with "Cue the music!" This, combined with the positive light Denny is always shown in.
Shows such as Revolution take place in Denny's head.This is a more general version of the above. Denny Crane seems to have reality-warping powers. Maybe a number of shows out there are fantasies that came from his mind.