Just heard an Adepticon interview with Ronnie Renton from Mantic, he diplomatically steered the conversation away from both a fleet scale Halo game as well as Vault stl options, so those two predictions look like they were spot on.
The initial release scheduled is for a small scale skirmish game, heavily influenced by their earlier game Deadzone, focusing mainly on Spartans and Elites, but Ronnie did say all of their plans are accounting for many additional game modes, including Grunts, so that is a slight positive. Possibly we will see boarding actions in the future, as opposed to replicating just the multiplayer experience.
