The key surprise element in the early X-Com games was the Mind Control thing, and you'd find out the hard way how your favorite killing machine operative was prone to alien mind control, and you'd have to fire him, if not do a kamikaze attack right on the battlefield. So, it got me thinking, what if each soldier had 2 hidden traits, and they would be revealed after a semi-random number of missions or kills, like, after 10-20 missions/kills. I envisioned these traits to be such as:
Diagnosed PTSD
Increased morale drop whenever a fellow soldier or a civilian is killed. You might want to think twice to send this guy to the Final Mission, whatever that may be in the future. Picture a dramatic verbal fight between this guy and his CO once he finds out he's not going be a part of the last battle.
Talented [weapon name] operator
Boost to the hit chance with one particular weapon (only if the weapon has been researched). At later game, you might get rookies with bonuses to newer weapons (since they've heard rumors of them and they can't wait to try them out), while you'd still have those seasoned machine gunners and such, adding flavor to the mix.
Dislikes [weapon type]
Slight penalty to the hit chance with all weapons of this type. Penalties to reaction fire. Note that this could be possibly circumvented by using certain firing modes or ammo types since they rely on various proficiencies.
Medic
Unusually skilled medic sometimes gets more healing done with the same amount of TUs. This chance goes up over time as the medic gains battle experience.
Alert gunner
Slight bonus to the hit chance when doing reaction fire. Slight bonus to the chance of doing reaction fire. Weapon is fired only if the hit chance is 70% or above, or whatever. Or possibly the trait triggers when an alien is just about to fire. Hit chances and reaction fire chances are increased when soldier is confirmed to have the perk.
Fast aim
Aliens have harder time interrupting a soldier who can wave his gun as if it were a pencil.
Hates [alien race]
Extra damage against a certain alien race since one such alien killed the soldier's brother / mistress / random nice lady next door / jaywalked. Can you picture a necklace made out of Ortnok toes? Needs autopsy research.
Fears [alien race]
Less likely to do reaction fire against a certain alien race, since he's seen them do inhumane things to human beings first hand. Smaller chance to hit these aliens. Severe to-hit penalties at close ranges.
Quarterback
Slightly increased throwing range and accuracy. Bonus gets bigger once the soldier is confirmed to have the trait.
Diagnosed antisocial behavior / dislikes crowds
Penalty to morale when in a team with 5 or more soldiers. Bonus to all skills when 4 or fewer members in the team. Trait would need to be checked again every time an agent dies.
Sniper
Option to spend more TUs to do an extra-aimed shot with select few weapons. With increased accuracy, of course.
Die hard
Generally more damage resistant than others. Faster health stat growth.
Nyctophobic (fear of the dark)
Unless the soldier is wearing infrared goggles, his morale is decreased, his aim is worse and he's more likely to do reaction fire during night missions.
Likes [operative name]
Soldiers can grow to like other soldiers. Increased accuracy when the buddy soldier is in the same team on a mission. Incur (recoverable) penalties if the buddy soldier dies.
Patriot
The soldier requires no upkeep as soon as the trait is discovered since he has donated his entire funds to PHALANX to fight the aliens. Instantly gain 10,000 credits or such.
Security specialist
Bonus to all skills during base defence missions.
Recon specialist
Lowered AP cost for infrared goggles as soon as the trait is discovered.