Hi there!
my merc ideas are not that punctual but might make the fight interesting.
Liquid gun:
My first merc ability idea is that the merc should have a gun that can shoot 2 kinds of liquid (with Q or E)
The first liquid would stick to walls and after a very short time they reached the ground or something would harden and create a bunch of spikes wich makes small damage over time . If a player is hit by it directly or standing near it when triggers would deal a higher amount of meele damage. This could block a passage or a small alleyway/door. The spikes however would be destroyable by explosives or a certian amount of bullets.
The other liquid would be a bit more jelly like and used as a deployable shield on walls or any flat surface. It could take more damage than the spiky one and would be a perfect cover for a short period of time,till the enemy runs out of bullets.It would only be destroyable via explosions so this could counter the mercs that has no explosive devices or airsupport.
Ice ability:
The other merc ability would be a bit similar, another liquid we can basically call fluid-ice. if the player shoots it on a ground it would make his and his teammates movement speed faster if they step on it. Its other cool use would be slowing enemy players down by shooting them with it. If you would shoot someone with it the enemy would freeze and wouldnt be able to move or shoot for a very short period of time.
You could even play hockey with them. Just shoot the ground,get behind an enemy,shoot him directly with the freezing gun and with a meele blow toss them around on the frozen floor. pretty fun eh? 
Balance:
There should be a Merc,who might wear a suit similar to the jet pilots pressure suit. His/her ability would be “Balancing”. His “pressure suit” would charge his chest up by pressing the “E” button,making him slower but gain more hitpoints. These hitpoints would not be the same as his health,but a completely different shield bar shown near the health. This would regenerate over time,in a similar way as health. With the button “Q” the charge would go down to his legs,speeding his running speed up by holding onto the muscles in on his legs and making his chest a smaller target. This also means the shield is partially or completely gone, depending on how much the button is pressed down. The longer one button is pressed and hold down,the more speed or shield he gains. This means you can also balance it out somwhere in the middle if you prefer that,but also change it,depending on the situation. If you have to get quickly to a C4 to defuse or defend just put all the suit energy to speed,but for defence the best choice is obviously the shield. I think this character would make quite an interesting experience for players and would be pretty fun!
Homing bullets:
Another idea I came up with is the "Cheater"or “Aimbot” merc. His ability would be the “homing bullets” or “heatseaker bullets”. Pressing the “E” button would make a circle appear around the crosshair. Every shot he makes after that would home in to the target within the circle. This homing ability would only be usable for a certain range,making it impossible to use as long range unfair sniping,but more effective at close to medium range.Bullets however would have an optimal range too wich means if the target is closer to you the more closer it should be to your crosshair,becouse the projectiles cant turn that fast to the target to reach him if its on the very side of the circle.(The closer he is the more accurate you have to be).
If the target is in a medium to longish range you dont have to be that accurate with your shots,they can home in and land on target easier but only in the circle. If the enemy is too far away the bullets would not be able to lock on to the target.
If there were multiple enemies in the circle the bullets would land on the target wich is more closely to your original crosshair.
This ability would not last too long…only as long as the phantoms ability maybe.You decide.
Itz would be hard not to make it too OP but can be a competitive,interesting character for those who are not too familiar in this very fast paced shooter game or just bad at aiming quickly.Could be also a starter merc maybe?
Thank you for reading!
Please forgive me for the bad english 
These are my ideas for now,but I’ll add more if something gets into my mind!
Hope you like it!
Fell free to make concept arts or add suggestions to my ideas,I always welcome helpful criticism.
(maybe I’ll add some concept art later on )
have a good day!
------UPDATE!-----
I have 2 new mercs as an idea!
First of all I have to mention,the reason why I don’t say what the mercs speed strength,nation or gendre should be is becouse I want to give the Devs the freedom to create their own immagination,based on these abilities and stuff.
Anyway,lets get into it:
Electric shock:
The first merc I was thinking of was the “zapper”, wich could have a lightning gun. This gun would be able to stun and damage enemies from medium to closer ranges. Also would be able to deal extra damage to players who arre under the effect of the previously mentioned freezing/ice ability of the other merc. By that I mean if the enemies are frozen or standing on ice they would get a bit greater damage than the others who dont. The difference between the freezing and the shocking ability is that the freezing would not allow players to move,just to turn around and shoot, however the shock would be the complete opposite,it would not allow the players to pull the trigger,but they could run away if they dont get distracted by the lightnings. The user of the lightning gun would get points if the teammates are shooting a player who is under the shocking effect.A bonus thing to this would be if the Shocking gun cold overwrite enemy mines,health stations or turrets,by turning them against its deployer!
Grapple:
The other idea I came up with is the grappling hook,which would allow the merc to travel a bit faster than the others,and pullobjects to himself. For exsample the player could grapple onto the ceiling and the gun would pull him up ther via rope or chain,but would drop him if he reaches it. Also would come in handy if you are in cover and out of ammo,but you see an ammobag laying on the floor,you cant get there,becouse they are shooting at you,so you just get the grapple and shoot the ammobag with it,pull it to yourself and now you have it without risking your life. The gun would automatically turn back to you if it cant reach any flat surface or grappling on a movable object like medpacks or ammobags. A nother cool thing would be if it could grab enemy players and pull them towards you,where you could knife them with a Khurkhi blade or dagger or something,but be careful,the enemies could shoot you while they are grabbed,but you have to hold the grapplegun so you wont be able to shoot back! Tricky to use but can be a very useful thing!
The reload time of this ability would be as much as far you shoot it,since you have to roll the rope up again.The closer you shoot,the less it takes to reload!
I suggest one of them should be like a french girl and the other could be an arabic man,like egyptian or something,since there is no mercs yet from these nations.
So thats it for now, I hope you like these new stuff!