Lemonysnippets asks an essential question:
"But if you’ve taken the friendship on the show as far as it possibly can go, if you’ve had them address that they are best friends, that they love each other, that they would die for each other and kill for each other, that they will remain best friends no matter what….and STILL they are both framed as being not only sad and unable to function well because of literal distance but also emotionally unsatisfied and sad and leaving things unsaid….
WHAT ELSE??? WHERE ELSE CAN IT GO?”
You’re in luck. I have some experience with this very conundrum, having spent nearly a decade of my life enfanned to X-Files. Let’s see… if they work this the way X-Files did, then…
1) John will get abducted for a long time, causing Sherlock to pine for him even more and get overprotective when John comes home.
2) John will shoot Sherlock to save his life.
3) The plot about Moriarty will get overgrown and convoluted beyond comprehension. They’ll put in some non-Moriarty shows, too… where Sherlock and John investigate Chupacabras, killer cats, and cannibalistic chicken companies.
4) Benedict will get tired of playing the character, so the producers will have him disappear (via abduction or some other plot device).
5) Concurrently, John will turn up pregnant. We will all assume the baby is Sherlock’s, but we won’t be sure when Johnlock was consummated, or when we moved into the Omegaverse.
6) Moftiss will replace Benedict with two newbie consulting detectives who are also attractive. Because people watch these shows for the PLOT, not the relationships and character development!