Book 2 is "Flight." Same starting point, but instead of responding by raising his own army, the crown prince type goes on the run. Instead of the war fantasy, the operative mode is the thriller novel, with the prince one step ahead of his pursuers. This time around, though, I play with the readers' knowledge of the first book, confounding it in some places, allowing them to predict events in others, by using the same cast of characters, but to different effect. Also include some revelations that shed new light on motivations in book 1. Both books end, somewhat anticlimatically, for a fantasy book, in tragedy.
The third book is "Or....?". There's a number of different approaches for this one. One would be to take literally, a number of different approaches, sketching out a plethora of alternatives for crown prince beyond those explored in the first two books, and using the readers' knowledge of the fictional world derived from the other two books to fill in the blanks. Or stretch out a single one, that deliberately incorporates parts of one and two. The real question is how far to make explicit the overall model in the last run. Should the narrator play coy about the repeated structure, or is that the point where they nod conspiratorially to the reader? Could the character be made aware of "the multiverse" without it feeling too hackneyed?
The idea of multiple time lines is pretty common in science fiction, and the retellings of such universes is pretty old hat--I mean, we've even got the "evil = beard" star trek cliche. I think it's a little more rare in fantasy. The closest is the "this story has many tellings" that you get through repeated variations on fairy tales and the like, or the approach where the fantasy world is deliberately constructed as a videogame, and is thus open to restarting and replaying with different approaches. (Vivian Van Velde's books are my childhood go-to for that sort of thing, though others have utilized it as well.) I guess Robert Jordan does it too, in some of the earlier Wheel of Time books, though not to this degree. All right, so it's a hack idea. I still think it would be a fun exercise in plotting. The trick would be to engage the reader in such a way that the repetition is a feature, not a bug.
Anyway, just an idea to toss out there. But it's mine, dammit, and if any of youse steals it... Well, then I'll have to think of a follow-up.