If Leona moves to a battlefield and stuns a unit, causing a legend buff that makes Fiora mighty, can Fiora then use her ganking ability to move to that battlefield?

Ruling: No, Fiora cannot use her ganking ability after Leona moves. All units moving to a battlefield must move simultaneously, and once Leona has completed her move, it is too late for Fiora to join even if she becomes mighty from the resulting trigger.

Sequence:

• Declare all units moving to the battlefield simultaneously
• Complete the move(s)
• Triggers (like Leona's stun or legend buffs) go on the chain
• Showdown begins after triggers resolve

Nuances:

• Standard moves can only occur when the turn is in an open state (no chain, no showdown)
• Even if a "when I move" trigger goes on the chain before showdown begins, there is no window between the trigger and showdown cleanup where another unit could perform a standard move
• Spells with move effects can be used during appropriate timing windows, but standard moves cannot