Xsyon doesn't advertise itself as a horror/zombie mmorpg, so I'll probably get flamed for this..
but I would love to see a lot more elements of the survival/horror atmosphere and mutated zombies as the toughest npcs. I know this game is really big on PVP, tribal warfare, and native american-like culture right now, but I could also see a parallel move in future development of zombies coming in from the outside and survivors having to band together to survive against them.
There aren't many zombie mmorpg out there, and I think this would draw a lot of new players in.
I too think this would be kind of a cool idea, if implimented in the right way.
What I invision as a cool transition would need a new combat system to be in place, but could evolve outward as the mist is lifted.
How about on one end of the map mutated animals start appearing on the edges. Weird, right? Yes, but not too suspicious. As the next level of mist is lifted we find that the mutations have effected humans. Suspicious.
In this way you could play it off life the zombies started on the west coast or the midwest and have their spawn points moving further inland.
This could be an idea for a second server (one implemented months, possibly years from now), one clean server for sandbox play and one server for event sandbox. These events could last a year at a time and could be used to see how the population as a whole survives the zombies invasion. The 'core/natural/normal' server could be used to see how people interact with eachother.