Someone always brings this up whenever a new sandbox MMO appears, and as I haven't seen it mentioned yet, I guess I'll do it. IN MMO's, there are core features, such as combat, crafting, harvesting, exploration, and travel. However, these features on their own can lead to players being bored after a while (like anything when done a lot), and some of the sandbox games that people remember most fondly (e.g. UO and pre-change SWG) had features of perhaps less important to the overall world, but that provided a kind of glue to the community, and some fun fluff type things to do in between the serious stuff.
The features I am talking about are things like having in-game taverns (hopefully player owned and run), card games in there or in other places, musician type skill perhaps, and other entertainment based endeavors that players can engage in with others. Some of these might lend themselves easily to Xsyon's core, like taverns. A tavern is just another building built by players, with a distinctive look perhaps and non instanced interior for many people. Others might be harder to implement, but having played a crop of recent sandbox games (Darkfall, MO, Roma Victor), they were all missing this fluff or glue that turns an empty world into a living, breathing entity. So I really hope Xsyon devs spend some time on this at some point, resources permitting.