It's many many many years down the road for our immortal warriors Vincent and Cloud. They wander the Earth as living WEAPONS, defending it at their own discretion.
Time has ground down everything the two once new. Wars and plate tectonics have changed the look of the very Earth. Disasters and cataclysmic events have restarted civilization again and again. And pockets where mako comes close to the surface have long since sank back deep into the Earth where no human could possibly dig deep enough to survive, and knowledge of both mako and materia have faded to legends and myths unrecognizable.
But that doesn't mean it's gone away.
Nowadays Cloud and Vincent both keep things under wraps in their own way, and with the sheer lack of magic around nowadays, they find they can sense it now in this near magic less void. Some of the only materia available now exist solely in the hands of the two WEAPONs, but occasionally, one or more crop up out of nowhere.
(insert whatever main plot you want here, whether it be a crossover with Harry Potter or Marvel or BTVS, or something more original and still within Final Fantasy)
What sparks off Cloud and Vincent's interest is finding a new summon materia. And finding this one seemingly sparks off more showing up, but the first catches the two's attention in a big way. For this first summon? It summons Sephiroth.
And for every consequent one showing up, it lets them summon more and more people from their original time, including things that were most important to them.
Worse though is that this fresh wave of summon materia cropping up, some of them have fallen into the hands of unscrupulous people, and the two WEAPONs will be damned before they let their friends be misused, let alone reveal and bring back the terrors of people materia and mako mining once again…
(As an aside, perhaps it'd be more interesting if Zack is the first summon materia found and Sephiroth is in the hands of the bad guy, though a reverse in which Sephiroth is on their side as a summon while they have to fight around their friends would be interesting too.)