You can get the following tasks from the initial field duty:
IV - Alli explosives, Horde mojo.
V - Scrying (Enchanting) Materials
VI - Arms (Alli swords, Horde axes)
VII - Armor Kits
X - Bandages
After completing one of those, you get a followup asignment, which can contain:
I - Desert sutvival kits (food, water, anti-venom)
II - Mithril boots
III - Grinding Stones
VIII - Uniform supplies (Mooncloth!)
IX - Extraordinary materials (enchanted leathers and bars)
XI - Skin of Shadows and Freyed Abo Stitchings for Mark of Remulos (for the Stalwart’s Battlegear quest at CC revered)
This is completely set and predictible, and it seems you have similar chance of geting any task from the list (any one isn’t significantly more likely than others)