Well we all know that the anime & manga tells us that Marco is dead and was somehow murdered through foul play during the Battle of Trost Arc. We also know that Annie somehow got involved with his death during the Female TItan Arc when Armin confronted her.
A lot of SnK fans have speculated that the corpse found by Jean was not of Marco's but another charact that looks similar to Marco, also part of the 104th Trainee Squad.
I agree that Marco isn't dead yet but a part of a larger conspiracy in the story that I think the author will reveal in the last 2015 or in 2016. The evidence I am pointing out is Marco's name and its meaning.
Marco means Messenger of the God of War. Now the wiki states that is his name because of the cruel fate that will be experienced bby the Traineed but I have another idea.
When we mean Messenger of the God of War in a literal point of view is that the person carries the meesage of the Supreme Being of War. The war between the Titans and Humans and we all know that there are people in the series who worship these Wall Titans as "gods".
And who was the one who initiated the war in the beginning of the series, the Titan Shifters, who are a class above the regular Titans. And we all know that Wall Titans and Titan Shifter skin has the same composition so it can be fairly safe to assume that these Titan Shifters are also considered as "gods" by the Wall Cult.
Messenger of the God of War. Messenger of the Wall Titans or TItan Shifters thus being a messenger of the Titans means that the messenger have an alliance with them. I believe that Marco is part of a larger conspiracy that will unfold. Probably he's been pulling all or some of the strings in the series primarily the actions of the Titan Shifters (Annie, Reiner, Bertholdt) there is a much more supreme TItan in "their homeland".
What do you guys think?