My game tries to be both a one-shot and a campaign at the same time. Each session is a complete episode but all sessions take place in the same game world where events and plots occur across sessions (and the world continues even when the players are on an adventure). NPCs will appear and reappear and they have their own motives and desires.
Unfortunately, the reality of OpenRPG and the Internet in general is that people are pretty inconsistent. People will sometimes show up and sometimes won't. Usually, a few people show up every session but they are usually not the same people. Since this is the reality, I try to make a single session complete and enjoyable, even if the player doesn't know the game background or what happened at previous sessions.
So far, players have not been consistent or inquisitive enough to know about or investigate the plots and threads that weave the sessions together. However, I have put considerable thought and design into having campaign-style human subraces, cultures, mass conflicts, history, subversive plots, wars, power plays, ambition, greed and fear. Hopefully, someone discovers those things.
I do my best to provide all these things. Hopefully, everybody finds enough of what they like to continue coming.