Considering this is Kojima's final metal gear (again), big boss and solid snake should both be in the game.
I don't know how, but they should.
Also, I want mg1, 2 and even mgs remade with this engine.
Kojima pretty much confirmed Solid Snake in an interview last year.

Edit: and let's not forget this Shinkawa art Kojima teased.

or the GameInformer hub cover

Solid Snake is in the game 100% and not just as a 12 year old child, fans have been asking for Solid Snakes return for years, this is the time.
Solid Snake voiced by David Hayter
#BELIEVE