For the final act, they ought to go back to the very basics: McClane on a ________. Only a modest budget necessary. And make him mortal again, for Pete's sake.
Has "cruise ship" been played out by Speed 2?
Maybe he'll pull a Stallone and make his version of Rocky Balboa (which together with the first one were awesome movies in themselves, the others are just brainless fun movies).
... Or maybe pull a New Nightmare
, and have the sixth and final Die Hard
star not McClane, but Willis caught in a McClane-like scenario?