Show me a picture of a "trone" and I'll tell you
Sorry. Maybe when Iroh was younger, but now that he is older, Zuko should take the throne with Iroh as his highest advisor on his court, and possibly as an ambassador to other nations. Iroh is so good-natured and wise, that would probably be the best use for him at his age.
Or, Iroh could serve on the military again, but after Lu Ten died, I don't think he would go for that.