My idea is, like, a bottle with a health potion, or a heart, which you can carry around, and then use on yourself or other players. (Possibly the user and the target player need to stand still for a second or two while it gets used. ... This whole thing is inspired by Left 4 Dead.)
#Healing items you can bring around
signal dome
frosty fiber
Problem is Action Bindung
On gamepad there is no free button anymore
Except Stick Press, which is awkward AF
brave cedar
On gamepad there is no free button anymore
Microphone
Yell "HEAL"
Or it could be accessible in the menu
signal dome
Maybe it falls under the domain of "tools" but without counting against the tool-limit, and you switch between it and the actual tools using the switch-between-tools button ... buuuut I can see that getting cumbersome, especially if there ends up being a tool-bag with three tools ...