I don't get it. The ICC is meant to be composed by judges who are experts in international law; kind of like the International Court of Justice. How will the judges be chosen? I don't know; perhaps by their peers around the world (or at least among the countries that have endorsed the ICC). It would need to be independant from politics. That's BTW, why in Europe, I don't think judges are anywhere elected by the citizen, but by a body constituted by their peers, by representatives of the parliament, by the senate, and others.
Those judges won't be accountable to countries but to international law.
International law already exists. But to judge individuals like the ICC is meant to do would indeed require other kinds of laws or jurisprudence.

Raoul