So there's been some talk recently about adding some verisimilitude and realism to the game, and I've been thinking about ways Pathfinder Online could be even more realistic. Then I thought: Why not diseases?
I used to have a really nice elderly lady as a neighbour. She'd talk over the fence sometimes when she wasn't doing anything else and her husband didn't want her inside - he stayed at home all day and she'd have to tend to him and cook dinners, that kind of thing. Later, it turned out her husband had actually been dead for years, and she had Alzheimer's; she'd just been imagining he was there and cooking roast chicken for an empty chair this whole time. She ended up being committed and ended up forgetting the faces of her friends and relatives, imagining that they were people she knew from the 1930s or something instead.
Now, what if that happened to players? Characters who have existed for more than a year or two start to find things changing - they see NPCs others don't, shopkeepers turn into quest NPCs from the tutorial level, quests disappear from your log as your character forgets to do them, skills start to drop... eventually, the character forgets how to swallow and dies. This would encourage players to make the most of the game and, tellingly, roleplay.
Other diseases could be modelled too. Cancer. Schizophrenia. Diarrhea. Some of these could be curable, to keep the game fun, if you pay a paladin (provided his alignment allows it). Not all of them, though; that would shatter immersion. We don't get to choose what illnesses we get, so why should our character?