I wonder why Annie was apologizing over Marco's death... Also, I find it suspicious how Reiner is always seen together with Annie in almost every episode making subtle comments about things.. o.O Maybe they have something to do with this?

If the theory mentioned by someone else previously that the armored titan and colossal titan are actually humans like Eren, it could be possible that they're characters that have at some point already been introduced to build on the storyline, rather than just randos.