When writing best-selling historical novels, it is not advised you conform to the mores of that era, because who wants to hear of old-fashioned manners no one understands or likes? If it's the Regency era you couldn't say dirty things in public either, and neither could a young man and young lady be alone in the same room and survive with their reputations intact. So, screw history! The heroine will be outspoken for that era and she is admired for it by the hero. The hero on the other hand will support women's rights and try to grope around as much as possible. Who wants to read about a long, complex, intricate courtship? Not the common market.
Sex and sensation always sells. Find some historical figure (say Anne Boleyn), point out a conspiracy theory e.g. hey Anne was a villain! Her sister was a good girl, and then describe what steamy goings-on went on in the court of Henry VIII. My history of the Tudors is non-existent so I can't elaborate. Just check out Philippa Gregory. But whatever conspiracy theory you do make sure it's full of sex and murder and incest if possible. Homosexual tendencies are encouraged as well. Oh, and the possibility that one monarch was illegitimate. Madness and disease is also an option. The King's Speech requires too much research and is struck off our list. Besides, that market has been explored. No, what we want is people in elaborate gowns and crowns.
Well, some conspiracy theorists say that Queen Victoria was illegitimate, and another group says she married, a second time, to her Scottish servant! You can try to depict the steamy scenes between Queen Vicky and the Scotsman, preferably with kilts and bagpipes, though I doubt kilts were popular in the Victorian era. A tear-wrecking scene involving the late lamented Prince Albert is required, as John Brown comforts the widowed Queen.
But this may be considered a trifle too prudish for the excited romance reader, because after all, it was the Victorian era, and Queen Victoria was a strict mother. Let us focus on a much-favoured era, the Regency. The Regent was known for his extravagances and his mistresses. He even attempted to divorce his wife, Princess Caroline of Brunswick. You could depict their stormy, raging passion (though he found her ugly, still it is a novelist's job to make things up) and their irreconcilable differences. Oh, there is a true anecdote. The Regent wrote to a friend saying he doubted Princess Caroline was a virgin on her wedding night because she told him his phallus was large. *Ahem ahem* You might then make up some imaginary lover she had before she married the Regent. Apparently after separating, it is said Princess Caroline had some lovers and an illegitimate child (she said it was the son of an old lady whom she was fond of, but who knows whether she paid off the old lady to come and say it was her child?) The Regent was by all accounts a tyrant. If you depict him as a Heathcliff, better still. Anyway Heathcliff is from the late 18th to the early 19th century, which is not very long before the Prince Regent's time.
But you get the idea. Make Princess Caroline appear a tragic battered woman. If you want to emphasise a tragic innocence, eliminate all the scandal surrounding her. Forget the historical records. They don't exist to a mass consumer public. If needed, just put in an orgy involving the Prince (and knowing his reputation, it shouldn't be too hard to find a real-life event) to the eyes of a horrified Princess.
Or something involving the actress the Prince Regent married illegally (it wasn't valid.) Romanticise it till the readers cry and want to kill Princess Caroline, who will of course become villified.
The Prince Regent's story is synonymous with his father George III the mad king. Now that's a blockbuster. He is a conflicted soul, troubled by marriage to someone he doesn't love and a mad father. I can envision a scenario where George III goes round with a stuffed chicken on his neck, swinging an antique sword trying to decapitate the entire palace.
But why write historical fiction? you ask, with all the research into the events. Well, the modern romance fiction market is overstocked, because sadly low IQ isn't as rare as we'd like to think. Marketing your novel as historical fiction shows you are more cerebral, because you have a history degree, and those snobs will be tempted to come down to read your novel. And those without a brain will have the pleasure of reading something "historical" and yet without any difficulty.
For something more speculative, try guessing the identity of the Dark Lady in Shakespeare's Sonnets.