Ground Zeroes/Phantom Pain

Ground zeroes is a prequel to Phantom pain set 9 years before, Big boss must infilitrate camp omega (Which is shown in the trailer) to resuce paz and chico, stuff happens and Big Boss lands in his coma.

Right now the confusing bit, the parts we are shown around the hospital and big boss with his horn and prosthetic arm, I'm quite sure this is during his coma. Seems like a very hideo-ish kind of thing to do seen as though there's lots of studies and debates about whether or not you dream in a coma and some people explaining that dreams in a coma feel like a parallel reality.

So ye stuff happens at the end he wakes up from his coma in 1984 and most likely the game carries on but big boss is different, the events that happen in his coma that felt real slowly start to change him. Hopefully the game covers a few years in his story, the gap between him waking up and creating outer heaven(Rescuing Frank Jaeger again and training sniper wolf) so it kind of completes the story as it's apprently going to big Hideos last game (We've heard this before)

Eli/Liquid

About the specualtion that Eli is Liquid here's a timeline:

1972 - Solid/Liquid were born

1974 - Peace Walker

1975 - Ground Zeroes 

1984 - Phantom Pain (Solid and Liquid are around 12, I think the advanced aging genetic changes don't kick in until later)

Eli is wearing a jacket similar to the one Liquid wears and he looks around the same age, As for people saying he was in the UK at this time:

"Initially difficult to categorize or recognize (dark, almost olive, skin and long white hair), and able to disguise his appearance in a number of ways, a cunning Liquid Snake spent a large percentage of his time as a mercenary and assassin viewing a career in the military with disdain. Previously, he had been engaging in real combat situations from the age of 13. His ability to blend in with almost any country's military forces allowed the British Information Agency (SIS) to install Liquid Snake as a sleeper in the Middle East. His mission parameters were simple -- accomplish espionage and destructive guerrilla activities. By the time he was 18, he was working for the British Special Forces (SAS), destroying mobile SCUD missile launchers in the Gulf War. After single handedly demolishing four launchers, he was caught by Iraq paratroopers, became a POW (Prisoner of War), and disappeared."

Big boss with horns

As stated above the this is during the time of his coma where he's in a weird lucid dream.