His most important potential role - for Iranians, if any - would be a symbolic one. The Islamic Revolution was and is THE most important moment in Iran's modern history. A literal black veil settled over that country in 1979, and the Shah represents all that was lost. His son serves as the current-day representation of that - or he could, merely from being the son (and he looks a lot like his father*). Tbh, I don't think there's a huge royalist appetite in Iran, and he doesn't seem to have been able to galvanize people much over the last 45 years, but he might just be the full circle symbol people will want. And a caretaker leader is maybe the path of least friction for an interim governmentt, and US influence there would be to our obvious benefit.