I'm surprised this thread hasn't been more active.
Things I'd like to see:
FFF-style write ups focused on various groups, showing how they interconnect. For example, one set might be a group of Ilviran pilgrims heading for Araka-Kalai, another might be the occupants of a small chantry or temple, town aldermen, the mangai council, or a patrol of one of the fighting orders. Basically, people who would be encountered as a group. This could be combined with Kerry's idea for a caravan if someone wanted to go that route.
Speaking of caravans, I'd like to see more about the various travel routes on Hârn. The Silver Way article was a nice overview of one of them. What about a similar treatment of some of the others? These could be the major ones, like the Genin Trail or the Salt Route, or maybe some of the more out-of-the way but intriguing ones, like "Trag's Lament" in Orbaal, or the trail that runs from Lorkin to Leriel. Or one or more of the Ilviran pilgrimage routes. It doesn't have to be all land travel, either. Coastal or riverine travel could make for an interesting article.
I'd like to see some short articles on the guilds, but also on some of the non-guilded occupations, like teamsters, scribes, or artists. There have been a few, like the Ratters article, but I think these would be great to see more of. Basically, take some occupation and explore it. Why would someone consider playing a character of that type? If it has an associated skill, maybe explore in more detail how that skill is used.
None of these ideas requires mapping, or even artwork. However, I'd be willing to bet that if someone came forward and said "I'm going to write an article about XXXX. Can anyone help with the art, mapping, etc" that they would get several volunteers.
Another great way for people to contribute fanon would be to create a collaborative effort similar to the collections of "Adventure Hooks." Pick a kingdom, settlement, wilderness, Atlas Harnica grid square, or city and write up some adventure outlines that can be used there. Or, if adventure hooks isn't your thing, collections of magic items, interesting books, potions, or other objects of interest. Is it something worth owning (or stealing)? There's probably an adventure to be developed around it.