Any ability as good as leech is should have some kind of significant drawback.
I believe Leech should have a -100 poison resistance drawback because you are soaking in everything around you, thus what might only be inhaled or felt by other players is straight up consumed by you.
Comeback Kid should restore you to 1 HP, and immediately make you immune to all forms of healing.
Breaking Sneak/Inviso is more difficult. Enemies should cast AoEs at where they *think* you are, which really just amounts to a highly randomized element where the computer decides whether it's your time to eat a fireball or not.
How the enemies would know where you are is based on hearing. Enemies that go inviso would have leave an aura that gives you a rough idea of where they are. They won't always be dead center or even in it, just near it. Creatures with superior hearing would be more accurate. Creatures with the "sniff" talent as well. Different surfaces either make this more or less obvious, as does the speed at which you walk.
