Old Kai will unlock it one day ha. Strike 2 so far though.
I mean his ultimate potential is probably mastered UI? Which doesn't really make any sense because Gohan would be there now. The fact that Gohan is currently stronger than he was in the Buu saga shows that Old Kai did not really unlock his full potential.
^ Whis postulated that when Goku resisted the energy of the Genki Dama, he "broke through the shell to the deeper potential within himself", and was able to access UI. ie. the Genki Dama was the trigger which allowed Goku to tap his latent potential/UI.
That's what makes me wonder if Old Kaioshin's unlock might have allowed him to attain UI as well..?
"We're all puppets, Laurie... I'm just a puppet who can see the strings."