in the cutscene following the combat between Dagon and Akatosh in Oblivion, the voice of Martin is heard talking to the Hero about Tamriel's future, as well as the Champion's; "When the next Elder Scroll is written, you shall be its scribe. The shape of the future, the fate of the Empire - these things, now, belong to you." Does anyone have any idea of what he meant by this?

It is a do what you want word so he is just saying that ES V is do what you want, you are who you are.

Also this should be in the Oblivion section

Not really, because it refers to the Next Elder Scroll.

It means, the person who wrote the elder scroll about the coming and going of the era of Oblivion and your story in Oblivion, is you.

moved in 3 2 1

no, this regards lore. The Elderscrolls can show infinite variations of an event until it happens and a hero "writes" it, when it becomes history the Elder Scroll only shows what happened.

It's a meta thing. You are the scribe (the player) of the elder scroll (the game). The events that happened are what you did when playing the game.