My idea about missions in DayZ is what we already know from ArmA2&3 servermissions.

Here is just a thought for DayZ:

Missions should depend on the ingame(real)time, approx. one hour before they will be canceled by the server. some character finds a item like a paper where the mission briefing is written on and the mission starts.

Mission:
Find a survivor(server AI) on a airliner crash site(placed for ex. anywhere around toploniki) and bring him to the hospital in elektro. if the survivor dies on the way to mission target, the mission is not resolved and canceled. if the mission is resolved, the player will earn special loot like nv goggles etc.
(This mission is dedicated to the movie "Warldwar Z":)

missions should be activated very rare in chernarus. maybe every hour a server mission in that area with a limitation to a maximum of 5(?) mission per server. the amount of zombies at the mission targets should be adjustet to the situation.

i think that this makes the feeling in the chernarus horror survival more fascinating, not just loot all the known lootspots all the time, in risk of getting shooted in the neck by somebody.

i also find that 5% of the houses in chernarus should be locked serverside with spezial loot in it. the keys can be found anywhere outlying of this village in a open house. this is what i call minimissions in Dayz.

This will bring the players back into the towns and villages away from the hotspots in the west.

the thing with the heli crash sites is already a step in the right way.

please let me know what u think about server missions.