Now that I've set the tone...
Someone commented that everything had been done in HnG fiction, which is why the fandom doesn't produce that much fic. I strongly disagree with that claim. There's plenty the HikaGo fandom can work with, and I think people would see that if they bothered to learn anything about Japan besides what they serve at their ramen shops - which, by the way, some people could still stand to research.
Sai is the focus of this entry because his story arc spans A THOUSAND YEARS. That's TEN CENTURIES, just in case you don't realize how much time that is to work with in the framework of Japanese history. He hid (lived? existed?) in Torajiro's go board for a hundred or two, but the rest? What did Sai do with the eight hundred years before that?
Big question there. Lots of room to work with.
Japanese mythology (and even history) provides both living people who are capable of seeing ghosts - priests, monks, onmyouji, immortals, extraordinary people who are none of the above but just have above-average perception - and also a host of supernatural creatures, gods, and spirits. These aren't just things writers make up for anime; they've existed in the Japanese consciousness for hundreds of years. There are plenty of people who could potentially interact with Sai besides Hikaru, both in history and the modern era. Surely you've all seen commercials for psychics, seen accounts of seances, and other such things?
In the Heian era, for example, there was a belief that the spirits of the dead could wreak havoc on their enemies in revenge for something that happened in life. I used Sugawara no Michizane as an example in an earlier locked entry - he was banished from the capitol and died in exile, but his honor was officially restored by the emperor because of a series of disasters that struck the capitol and were blamed on his angry spirit.
That could be a Sai story right there.
And there's the history itself. Sai wandered for a thousand years. That's a long bloody time, and in that time the culture he grew up in was was plunged into war (see the Genpei War, which has its own body of legends) and reborn as a warrior era with Japan's very first shogunate (see the Kamakura Shogunate). This government reformed itself every two or three centuries, coming under the control of different families and so forth, so we have the Ashikaga Shogunate, the Warring States period, and the Tokugawa Shogunate, which is probably what most people think of when they see the term 'samurai.'
Go was present throughout this turbulent history. There's the development of Zen Buddhism to consider. How about the emergence of art forms like No, Kabuki, tea, et cetera? Besides historical possibilities, there are AU possibilities. Also, there are probably shades of Akira and his family, for instance, that aren't touched on that often in fic - family dynamic, family tradition, et cetera. I don't actually know, as I haven't seriously read HnG fic in a little while.
And please: Sai had a life. Probably a very interesting life. He looks like he was in his twenties or so, by the artwork. By then he would have been married, and he had clearly risen to a prestigious position in court, so what is there NOT to talk about? Think of the political potential, the affairs he could have--
--except, people seem to like the idea of Sai as a blushing virgin. Well, if you ever change your minds, I'm pretty sure that by that age, he'd have known quite a bit about romancing women and men. He must have had at least one wife, and even if you don't buy into the idea of Heian men as complete skanks across the board (I do), it's unlikely he was innocent regarding sex, the exploits of other people, or whatever.
Also, Torajiro? Anybody? They were together for twenty years - more. The Wikipedia entry on Shuusaku provides plenty of fodder for fic. Their relationship has plenty of room for writing, too.
I've wondered for years what has stopped people from writing this stuff when it seems everybody loves Sai - or at the very least, there are always people who want to see more of him. You don't need to write a resurrection fic to do that. There's plenty of Sai to go around.
[ETA: link to conversation found and added.]