Spacetime.
It was one of the theorized Six Paths of the Mind that Kaizer developed, and was something he personally was looking forward to unlocking because of its sheer power and utility. Look at the Dean, and look at Sophia, able to be virtually immune from danger as they could enter the void and even come and go as they pleased.
The Dean even developed a plan for the six paths of the mind, emphasizing that Kaizer should focus on unlocking elemental control and mastering the fire, water, and wind elements before moving on to Channeling.
Spacetime was the last thing on his plate, and the Dean even developed a detailed plan involving personal training to help Kaizer master it, involving himself and Sophia, which was why he even connected his daughter with Kaizer.
