Rob Doherty,
in this interview, says there was some thought about killing either Sherlock or Joan in the finale:
For me, the "moxie" ending would have been...
Cut to five years later. Sussex. A small-ish English house near the coast. Mid-morning, on a bright Sunday in early summer.
The camera zooms in. It's a man, tending to beehives. As we close, we see that it's Sherlock. We see that he's not alone. There's a small child with him, about three years old, holding his hand. We stay with them for a moment, and in the background we see another figure on the house's back porch.
This figure is seated, wearing a wide-brimmed hat and with their back to the camera. The figure appears to be facing an easal. Is the person painting?
The person is painting. Stops. Stands.
"Sherlock, darling, it's time for brunch."
Sherlock gives a tetchy nod. "We'll be along presently."
She smiles. Sherlock and the child walk back to the house.
"And what were you showing our son?"
She kneels down, as parents do, to call their small children to them. The boy's hands slips from Sherlock's and he races across the grass.
"The new queen will be born tomorrow or the day after. Young Mycroft is particularly keen to see it."
She stands. They embrace, and Sherlock Holmes and Jamie Moriarty walk into their Sussex cottage, hand in hand, with their son.
Fade to black.
I would not have hated that ending. Teel the truth, I would have liked it a lot. I don't dislike the ending we got at all, but there's also a part of me that wanted to see Sherlock retired on the Sussex Downs, and even those two crazy kids working it out.
With the time jumps, CBS could make a reunion movie in 2025, and it would only be a year or two after the end of the series.