1. Hercule Poirot
Created by: Literally the best-selling novelist of all time, Agatha Christie, whose novels have sold over 4 billion copies and whose 1952 play The Mousetrap is the world's longest running piece of theatre. Her best loved character, egg headed (both literally and figuratively) detective Hercule Poirot made his debut in The Mysterious Affair at Styles in 1920. The book was rejected by major publishers and it was not until 1926's The Murder of Roger Ackroyd that Poirot became a household name. In total Christie wrote 33 novels and more than 50 stories featuring Poirot before his death in 1975's Curtain (as well as 12 novels starring elderly spinster Miss Marple). Like Arthur Conan Doyle, Christie tired of her detective creation long before the public did, describing Poirot as: "a bombastic, tiresome, ego-centric little creep".
What's his appeal?: OK, I'm aware that the purpose of this article is to suggest classic detectives to replace the finally finished Poirot series, but who better to replace Poirot than Poirot? If audiences can get excited for three simultaneous Sherlock Holmeses (Downey, Cumberbatch and Miller), then another Poirot is a definite possibility. Poirot is second only to Holmes when it comes to detectives with instantly recognisable style. A 21st century Poirot could, like the BBC's Sherlock, bring him into the modern day whilst retaining everything from his fastidious personal grooming and neatness to his obssession with "the little grey cells". It's important to maintain the character's status as a foreign observer, a police detective in his own country, a nation whose war torn destruction has caused him to flee to Britain and a job as a private detective. These elements are more key than being Belgian specifically and open the door potentially to a more racially diverse cast and an African Poirot. That way he could maintain the characters Francophone tones whilst still being indignantly "not French".
Sidekicks and supporting cast: Captain Arthur Hastings (or Lieutenant at first), Inspector (later Chief Inspector) James Harold Japp and Poirot's secretary Miss Lemon are largely analgous to Holmes' Dr. Watson, Inspector Lestrade and Mrs Hudson, something that was fairly deliberate on Christie's part given how indebted she was to the tropes of her mystery writing predecessors in the early days. As the likes of Martin Freeman or Lucy Liu's Watson or Rupert Graves' Lestrade show, these supporting character tropes are easy to play with and adapt for new versions that stay true to elements of the originals.
Any on screen previous?: The ITV-Suchet series is far from the first version of the moustachioed Belgian on screen and it will almost certainly not be the last. Perhaps the best known version was Albert Finney's Oscar nominated turn in Sidney Lumet's 1974 film of Murder on the Orient Express. Finney's Poirot appeared opposite an all-star cast including Lauren Bacall, Ingrid Bergman, Sean Connery and John Gielgud. Poirot was once again surrounded by stars, this time including David Niven, Maggie Smith, Bette Davis and Mia Farrow, for 1978's Death on the Nile in which he was played by Peter Ustinov. Ustinov, never the most faithful interpretation of the character, would go on to appear as Poirot in two more all-star adaptations for the big screen as well as three TV movies updated to the present (one of which featured Suchet as Inspector Japp). Just this year, as the Suchet TV series finished, Ridley Scott was attached to produce a new film version of Murder on the Orient Express, although whether it actually gets made along with all of Scott's other mooted projects remains to be seen. It does, however, indicate a continued appetite for new versions of the famous detective on screen.
If they only made one: The previously mentioned Roger Ackroyd is generally regarded both as Christie's masterpiece and as the great novel of mystery's Golden Age. However, its ingenious twist - the Watson/Hastings type narrator turns out to be the killer - has meant that there has never been an entirely satisfactory screen version, including the Suchet version. Christie's novel is a masterful piece of editing and ommision. Everything that Dr James Sheppard reports is accurate and with a documentary reliability such that the audience never questions whether we might be being given the truth and nothing but the truth, but perhaps not the whole truth. Conventional screen narratives struggle with this kind of story structure, but it would lend itself perfectly to a mockumentary or found footage style in which the film is Sheppard's assembled footage of events. Not only could this be a creative take on a classic story, but would also make a salient point about the subjectivity of factual documentary footage.