File:Isaac Mechanics Prototype and Sample Combo

Isaac's proposed mechanic is time travel. http://www.labzerogames.com/3rdcharacter In this example, a double appears from the future with an attack, and after five seconds have gone by Isaac returns to the past to deliver the attack. If the double is hit, Isaac takes the damage when he returns to the past, and if Isaac is prevented from returning to the past the enemy regains any life they lost from the attack. The double's attack has no startup or recovery on Isaac himself and hits instantly, allowing it to be used completely without fear.

In a real version of this, Isaac might have a super where he disappears into time then reappears. He could use this to travel forward in time, giving himself a "free" double later. Or perhaps he could use it during the countdown, to eliminate the forced trip to the past.