The Murder of Roger Ackroyd would be a difficult choice, mainly because the medium of film would make it impossible to maintain the twist made the book so notable that the first-person narrator is the murderer. Still, it could be done, if Branagh and the screenwriters are up for a challenge; Poirot brought it to television.
Jossed. The third film has been confirmed to be an adaptation of Hallowe'en Party.
Poirot's moustache
At the conclusion of this film, Poirot shaved off his moustache, essentially renouncing the vanity and ego that indirectly led to the death of his friend and the ruination of possible friendships. However, the next case, if handled properly, could see him decide to regrow said moustache while approaching his old attitude from a new angle, much like how Captain America temporarily adopted the identity of Nomad after he lost faith in America's politics before realising he could stand for its ideals.
If Simon had gotten away with killing Linnet
Jacqueline would have been his next victim. A man who is willing to marry, then kill, a woman for her money isn't going to want to share his ill-gotten gains, especially not with the co-conspirator who could turn on his at any time if things ever went sour. And Jacqueline, by playing the Stalker with a Crush role, would have given him adequate reason to do so later—he could easily claim that she came after him *again* with a gun, and he killed her in self-defense.
