Here is my prediction for the teaser from Kojima
First off, the teaser shows 2 characters. The first is a character that looks like Big Boss, but not old.
The first thing to note is his age, this image would put him in his late 30s or early / mid 40s.
Things get interesting when we look at the 2nd character. It looks like Raiden at first glance but a couple things stood out for me. On close inspection, the eyes really got to me. The color is not the same as Raiden's! Looking further I find that the hair, facial features, and the eyes match perfectly to Sunny! If you remember, Sunny's mother was the Ninja Grey Fox in MGS2 but was killed later in that game. So if we believe for a minute that this is in fact an older Sunny, then we must revisit who the "Big Boss" type character is. My thought? Johnny "Akiba" looked stunningly like a very young Solid Snake. Being a genetically altered soldier that lacks nano machines, it is easy to see how Johnny could be another Naked Snake clone without the complete knowledge from the patriots. Johnny stuck out like a sore thumb in the last game but it was never fully explained.