Common sense would dictate it will be an expansion on the existing platform, as I cannot imagine IO developing a whole new engine for Season 2. Then again, I still believe the game was intentionally broken in half in order to sell a season one & season two separately, which is what guides my belief that season 2 will be an expansion.
Selling 'season 2' as a completely separate game makes no sense. If that's the case, why not just market it as a sequel? For example, Gears of War 2 was the sequel to GOW, not the second 'season', although it was a continuation of the first's story and character arcs. Unless the label of 'season' is being used instead of 'game' to justify an incomplete story / lacklustre content, as consumers will (foolhardily) believe their experience will be enriched upon release of future content.
This whole situation is shady as fuck (very shady).