I'm not going to write out my whole mission here, but I also didn't want to start three separate threads for different ideas I had, so I'm putting all the rest of my concepts not discussed elsewhere (IE Faking a Psyker,) here.
My mission plan takes place in the desert, and so to avoid the most cliche tropes I thought it would be interesting to make the desert cool and safe in the daytime, but hot and dangerous at night. So, here's my plan: The sun is fairly distant, and does little to warm the planet, but their moon is radioactive. I'd add some sci-fi-mumbo-jumbo to explain that the solar rays counteracted the mild radiation during the daytime, but whenever the moon was out and the sun was down, the desert becomes a dangerous wasteland unless you're under cover.
The rules I worked out were:
During the day, the temperature is a cool 65F, but at night it raises to well over 100F.
For ever hour you're in the open desert at night-time, you take one point of Toughness and Strength damage.
If under some kind of covering (A tent or similar) this damage is halved.
If under protection designed to keep you safe from radiation, (IE a rad-tent) this damage is reduced to 1 point per night.
Lastly, if in a permanent habitat (A mansion, inn, tavern, etc.) it's assumed to have radiation shielding and no damage is taken.
This damage heals normally, but you have to stay out of radiation for 24 hours before you begin to heal.
Does this seem fair? Or cool? Or even something you'd touch with a ten foot pole?
(This is the one I'm least confident about.)
Essentially, the resolution of the plot is a hunt for treasure. After reconnecting and fixing the electronic map, it will lead you to the location of an ancient Space Marine relic. (The mission was not actually to find the relic, and therefore it's loot, the mission was just to find a man. The man, in turn, had fled to the desert looking for the relic.)
The relic was actually part of a Space Marine Psyker's armor, countless centuries ago, on the same planet as the desert. He was vastly powerful, and helped fight off the chaotic forces stationed there, eventually paving the way for the planet to be colonized by imperials. However, in one of the final battles, his life was lost as he was crushed by a Daemon Prince... But not before delivering one final blow to banish the Daemon back to where it came from. His body was presumed lost, and indeed most of it was. All that remained was the battered, weathered gauntlet from the arm which had wielded his force axe. This isn't too important, it's just how the relic got there. Anyways, over the years it was worn down, mistreated, stolen, lost, and rebuilt, but it still contains a spark of the power from its original owner. If worn, it gives 3 points of armor to the left arm, and allows the wielder to test Willpower to resist Psychic Powers. (If the wielder can already do so, they get a +20 instead. If the wielder is a Psyker, they instead get a +2 to manifest.) Additionally, the Gauntlet gives one Fate Point per session which cannot be burnt, but can be spent just like any other. (Blessed by the Emperor and what-not.)
This relic has been in a cursed town for the past five hundred years, however, its own righteous energies being beaten back by another relic. This is where I need advice... I was vaguely inspired by Halo Devices here, and this is what I had in mind:
The evil relic (Some kind of helm or crown) suppresses all Psychic Energies, essentially counteracting the effects of the Gauntlet. However, it's own evil nature also suppresses the will and humanity of those around it, slowly turning them into mere shells of their former selves. If worn, the bearer becomes inhumanly strong and durable, and those around the bearer experience a far more minor version of these mutations and effects. I don't know what buffs are fair against level 4 and 5 players, though.
The idea is that the relic was placed on the planet to begin sewing corruption and insanity, so that the planet would fall from the inside and be ripe for invasion by evil forces. However, it found it's way into a small village in the desert where a group of pilgrims had taken to worshiping the gauntlet as a relic of the Imperium. The Gauntlet's power and spirit were able to shield the rest of the the planet, suppressing the effects of the evil relic to the town alone... And the townsfolk were driven insane and mutated into inhuman beasts. The townsfolk now believe that both relics are gifts from the emperor, and that one day a great warrior will come along who can wield both into battle, the mind and fist of their deity, crushing all opposition. In reality, wielding both is impossible without being utterly consumed and quickly dying from the conflicting forces.
Does any of this make sense? I tend to ramble a bit after I've had a few vodka gimlets... ;P