And he's pull off well slightly sinister or even a bit deranged parts before. Any of the remaining Greyjoys are certinly possibilities.
We'd need to know if keeping his part secret was deliberate (and hinting his character is a spoiler to non-readers) or if the announcement he joined the GoT cast just leaked ahead of time and HBO will later announce his part in the official announcement. Because there's little reason to keep his part secret if he's playing a Greyjoy after the s3 finale just re introduced them in the story..