It depends mainly on the fact, F2P or P2P, because there isn't much to do in F2P, but there are a lot more opportunities in P2P, which not everyone could afford. In F2P, you could have a race from Lumbridge to like Draynor or Edgeville, or you could have a race from Falador to Karajma, who knows.
I can only really see races in F2P, but P2P you could have agility races, or Castle Wars.
Or F2P you could have a mega duel tournament, levels would need to be balanced though.
I don't know, it'd be difficult to have a Runescape event that wouldn't be one-sided.