If it's not at the event in City of the Ancients, I expect it to be at the Northern Crater, with the character being frozen in mako not being Sephiroth this time, leaving a huge twist to be resolved for the third game and basically confirming the Rebuild of VII.
Instead of Sephiroth, who we already know that might be the OG one and he's not frozen in the crater, Cloud will find another Aerith, after the RE Aerith follows the fate of the original in City of the Ancients, making the player think that at the end, event won't change, until this reveal