I think this would be a terrible idea!
Conceal certainly helps in those pure stealth duo shinobi runs but moving it to MSgt would hurt more than just those stealth missions. In small squad missions - those 5-6 men missions a lot of players desperately want to see more frequent - Conceal is the only option for a Shinobi (apart from being an officer and using command) to help the squad without giving up his scouting function. Those missions usually have one round or two where you need to throw all the firepower you have at the enemies. You can't afford to have 1/5 of your squad sit idle doing some scouting (something you can easily do in 10 men raids and of course pure stealth missions). The real value of Conceal is in those 5 men missions, not in stealth runs and removing it or moving it all the way to MSgt would probably hurt those 5 men missions much more than the stealth runs or 10 men raids!
I don't even think Stealth missions are broken and need fixing, i think what we need is some incentive to do 5-6 men missions instead when infiltration timer allows it. However if really the developers want to "fix" the abuse of conceal on stealth hacking missions here is an alternative change that wouldn't hurt the normal use of conceal in fight missions. Change Conceal so that :
- It can't be used if an enemy has LoS on you (even if you have some cover) : no more vanishing in front of the enemy after you've done your job. In normal fighting missions you'll just conceal at the end of the battle and contribute to the fight as long as there are some enemies, it will only prevent using conceal for a second Shadowstrike shot in the same fight.
- It can't be used if you are carrying something : object from a chest, hacking data, unconscious dark VIP (last one is particularly silly). This would completely stop it from being the absolute answer to stealing something in the middle of several pods. From a realistic PoV just consider whatever object you stole has some tracer that can be removed easily in a couple of minutes inside the skyranger but not in the heat of action.
- It can't be used while an evac flare is "glowing" (during the time between calling evac and the skyranger arrival). This one would fix the issue explained by Jadiel in post #2 but would affect non stealth missions so would be a much harder hit on the ability as pre-placing the evac flare can be a good idea in fight heavy missions. Maybe simply prevent the guy who called for evac from concealing if it's possible (if calling for evac adds a flag to the soldier object that the game can access), otherwise maybe use a cooldown to prevent concealing immediately after someone calls for evac.
Nothing can be done for extraction missions as Conceal isn't even used in those. A shinobi could still conceal after hacking a cell door but the VIPs will never be concealed. Assassinating a VIP with a shinobi and then running away to conceal wouldn't be easy anyway and would be a really stylish approach to those missions compared to suicide runs. Even planting X-4 and running away would be harder as there is often a pod or 2 patrolling near the objective and being unable to re-conceal when they see you would hurt in those missions. Conceal really only seem OP on hacking guerilla missions but it's a valuable tool in missions we certainly don't want to hurt more than they already are - look at xwynns video#77 to see how badly a 5 men run can end if you don't quickly dispatch everything!