The X-Men's Iceman has a forgotten secret super-power - shapeshifting. One of the original X-Men, Bobby Drake is among the most powerful mutants to walk the Earth. He's an Omega Level mutant, recently defined in Jonathan Hickman's X-Men run as someone whose "dominant power is deemed to register -- or reach -- an undefinable upper limit of that power's specific classification."
Iceman has traditionally been reluctant to use his powers to their greatest extent. In 1986's X-Factor annual, a ruthless scientist who called himself Doppelganger duplicated Iceman's powers, and immediately demonstrated that Bobby had powers he had never imagined; he transformed into a fully-fledged ice-shape and actually began to alter his physical appearance. Iceman still beat Doppelganger as the scientist's creativity was no match for Bobby's experience, and over the next few years, Iceman began to experiment with shapeshifting.
In Uncanny X-Men #314, Iceman's body was possessed by the mind of Emma Frost. Emma had been left in a coma after a battle with the Sentinels, and she cared little for returning to her own body, instead concerned primarily with finding out what had happened to her students, the Hellions. She escaped the X-Mansion, where her comatose body had been treated by the X-Men, and raced to New York. What's more, like Doppelganger, she showed an intuitive grasp of Bobby's abilities. She too transformed into a full ice-form, and it was resculpted into an androgynous, quasi-human form. It was only when Emma attempted to return to human form that she realized she had unlocked a secret power; the potential for shapeshifting.
When Iceman reverts to human form, it seems his subconscious mind controls the transformation, ensuring he returns to normal. Without that, though, his body would re-form itself into the shape his ice-body had taken. But what would happen if Iceman consciously chose to override that process? Uncanny X-Men #314 clearly sets the precedent that Bobby can morph into any shape he wishes. The process won't necessarily be painless, as Emma Frost found the experience agonizing, but to be fair she was unwittingly attempting to revert to some sort of quasi-human shape, meaning her very bones were attempting to reknit themselves. Furthermore, there's no reason to suppose it couldn't become easier with repetition.
Iceman is one of the most powerful mutants on the planet, and his ice-powers essentially mean he can manipulate the entire world's atmosphere. But Uncanny X-Men #314 reveals there is another element of his powers, one that has never been explored since; he has the potential to be a shapeshifter as well. It will be fascinating to see if any X-Men writers toy with the idea further down the line.