This is an item that, once used, revives your teammate. Obviously, it's only applicable to team-based game modes. Specifics: Must be very close to your teammate to use. Like touching them close. Effective only during the next turn. Restores teammates health to full, but not including armor (blue bar). Player can only be revived once per game. If you think this is too unfair for those who don't have a revive item, then it could instead be a game setting, and not an item. The same rules would apply. You would have to get close to your teammate then click a revive button. You wouldn't be able to shoot and revive in the same turn.
Full HP is excessive, that could change the game's course too drastically, but something like half of original maybe.
Possibly reviving them to half hp like mentioned already or letting them be restored with full armor only (however much they have to start) would be cool.
Weapon: Rebuild Unlocked by secret achievement: Kill 500 players on your team This weapon only works if a player is dead. Launches a small, normal shot sized projectile which, when it hits something, will restore 10% total health and 5% total armor. Works on both teammates and enemies. Level 2: Restore Upgraded with 1,500 EXP The projectile is the same size, but it now restores 25% total HP and 10% total armor. Level 3: Revive Upgraded with 10,000 EXP The projectile is slightly larger and now restores 50% total HP and 20% total armor.
What I mean by secret achievement it that it does not say what you do to unlock the achievement. There is just a picture of three question marks. So no one would know that you have to kill your teammates in order get it. Its meant for those accidental kills.
How about instead of reviving them to full hp, you take some of the hp you have and transfer it to your teammate (Maybe around 20 - 30%)? Or you could transfer the armor that you currently have into their hp. I'm basing this around give and take, so I'm just giving an idea of mine