just my two cents but here is how it think it will turn out.
Solid Snake will be in the game 100% sure. Kojima kind of teased him in an interview with CVG.
http://www.computerandvideogames.com/414360/interviews/e3-interview-kojima-on-mgs-5/
First he showed young Liquid (Eli) in the red band trailer and now (E3 trailer 2014) i think we'll see young Solid.
Solid Snake will be voiced by David Hayter. (not the young one) Later we will play as Solid Snake and fight against Big Boss (Kiefer Sutherland). The reason why they let Hayter in the dark for Ground Zeroes is because The Phantom Pain won't come out for a while so there is no need to tell him just yet.
Solid Snake against Big Boss would be pretty stupid to have the same VA so yeah...