I'm (slowly, slowly) working on a campaign I'd like to run for my friends one day where the PCs are part of a fairly traditional fantasy world, but during the first session they'd gain the ability to jump a few hundred years to an advanced, magitek version of the same world and back as a result of being in the right place at the right time when the BBEG from *that* time first jumped *back*. They'd have to fight a war on two fronts because if they lose the present, a future where they have a chance will never come to pass, and if they lose the future the BBEG will gain enough power to be uns…