It just released today, has any one bought a copy yet? It's a direct sequel to Glasslands.
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I think if Traviss tried that the books would be three times longer and I suspect a few folk would be confused by having multiple factions on both sides. Personally, I'd like it. But I don't think 343i would go down that route, even if both sides are (theoretically) capable. It might be mentioned one day as happening.