Well if you think about it, Wizards of the coast are trying to go back to
roots as far as story telling goes in the MTG universe. So just think on what HAS happened, what is OCCURRING, and what effects those actions may have on the future. For example,
Gideon realizes just how vulnerable he really is ink the universe. Maybe he goes back home to Theros and gains a new color by fighting
Ashiok, nightmare weaver. That would be traumatic for Gideon if he had to face the shades,, or rather, the "Ghosts of his pasts".
Or perhaps Nissa goes back home to Zendikar or to Shandalar by accident and runs into Garruk. That would be an interesting encounter. The protector of world hearts vs. The primal
force of nature.