WMG / Hercule Poirot

Hercule Poirot is the ancestor of Adrian Monk
Two Words: Super OCD. Perhaps Poirot and Countess Vera Rossakoff had more than a little admiration for each other...

We know from canon that Sherlock Holmes had French relatives; could he, perhaps, have had BELGIAN relatives? Perhaps that is why Poirot didn't go back to Belgium after the war, if he had relatives, albeit distant ones, in Britain.

The detective we see on screen is not Poirot
Whoever he is, he refers to Poirot in the third person. Is it possible that Poirot is really like Mycroft Holmes, a figure hidden from the world and only in contact via an accomplice?

Poirot wears a fake mustache in Curtain because he lost the original in The Big Four
After faking his death due to a explosion Poirot disguised himself as Achille in that disguise he had a scar in a location where it would impossible to grow a mustache as such he'd obviously have to wear a fake mustache for the rest of his life just like he did during the first part of his Achille act, that's why he had a fake mustache in Curtain.

Achille Poirot actually exists
In The Labours of Hercules, Dr. Burton remarks on the odd naming convention that Madame Poirot gave to her sons, and mentions Achille's name. Most likely, Achille Poirot did exist, but had presumably died at some point in the past. Hercule is simply borrowing his brother's identity in ''The Big Four"

Poirot was Norton's final pawn in his stream of murders by proxy.
When Poirot presented his belief that Norton was murdering by proxy, Norton smirked and seemed unconcerned by Poirot's plan to execute him. Norto was planning on corrupting the Great Hercule Poirot into a Murderer from the beginning!

The Darwin Awards have an entry for Mlle. Blanche
That series of murders was all over the headlines, and blackmailing a murderer? Seriously?

Hercule Poirot is a Time Lord
It would explain his curiously extended career and very high intelligence (and perhaps some of his quirks). Furthermore, by this WMG, Hastings, Japp, and Miss Lemon are his companions. After Curtain, he left earth to carry his trade to the wider space-time continuum.