Dunno, the Vodka idea stated by some SA seems like a poorly-recieved joke.
The only logical ones left seem:
Pure Cacao (with a strong bitter Cacao bean like in L'Instant)
and one with a Werther's topnote, called Pure Toffee/Pure Caramel/etc.
He could always just get stupid and run the gamut with less intriguing:
Pure Vanilla, Pure Mint, Pure Patchouli, Pure Cognac (although Malt definitely had some grape/fruit nuance already)
My guess is Pure Cacao, it would do better sales wise than another.
Especially if it is very bitter cacao, people will go in thinking "oh, it'll be more sweet chocalate", and BAM, bitter raw Cacao.