As if we don’t have enough of these… I do think that there’s not enough variety of perks in game however, so hopefully one or two will get picked up.
Faster Strength/Will/Speed growth - This could be a low level perk that then reduces the cost of training for the respective stat.
Lone Wolf - When no other units are near, this soldier will have a enhanced ability to be stealthy and/or aim.
Night Wolf - Increased ability to be stealthy during night missions.
Urban Wolf - (You can probably tell where this is going… some soldiers could be specialists for different environments)
Tunnel Rat - Increased movement/aim when attacking alien nests.
Better Hearing - More accuracy/range to hear your opponents’ movements.
Jab - A lower AP cost for melee hits/less damage.
Last Ditch - Increased melee damage when health is low.
Ugly as hell - Melee attacks will also reduce will.
Get ready for it - For those moments where you know what is coming around the corner, and where you want to hit it. The sniper can take the time to choose a target when firing from Overwatch.
Increased Overwatch angle - A possibility to set overwatch in a wider arc than is standard.
Too busy to bleed - Reduced bleeding cost OR bleeding automatically stops after a couple of turns.
Mithridate - Reduced poison cost OR poison automatically stops after a couple of turns.
Longer shot burst - Firing automatic weapons will use an extra bullet or two for each burst (the downside being that you’ll reload more often)
Reduced recoil - After the 1st bullet, following bullets will have less divergence from that 1st bullet.
What are you? - I don’t think you should automatically know the enemies’ health/armour value etc, but maybe a perk could let you assess it.
Slinger - Less AP cost/reduced accuracy for throwing a grenade.
Let it cook - Ability to throw a grenade that will explode on the end of the opponent’s turn.
Demolition man - Increased damage when shooting at structures.
Finishing shot - The ability to fire off a single bullet rather than a burst. (0 willpower cost for this one)
Full Auto Wide burst - The ability to set a start and end point for firing, and then unleash a full clip between those two spots.