Welcome to iMore. Do not know. Only Apple knows for sure. My guess would be IOS 9 at least, if not IOS 10. It will depend on where Apple sits with development.
If a March launch holds true, my guess is 9.2.1 only because it's really close timing with a possible 9.3 GM in the near future. Unless it launches alongside iPhone 7 (doubtful) iOS isn't in the deck of cards.