I haven't thought much about it, but it definitely invokes a solo fight between Robin and Marth at the Nohrian stadium.
So it starts off with Marth staring down the Avatar with his weapon ready for a fight. Robin describes how ancient his style is while taking note of his as an old warrior. This would cause the Hero King to clash swords with Robin on the stadium. Which would results in the water having a rippling effect. Marth would try to attack Robin with another slash after withdrawing his blade. But the skilled mage would then use the Thoron spell to scare him off, only for the Hero King to lunge at him for another attack. Robin would take his Levin Sword out only for it to be easily started by Marth's attack.
This is where things get really interesting since the Avatar would be kicked away by Marth. Wounded and heavy overwhelmed, Robin was now open for the Hero King to finish him off. In dramatic flair, Marth would raise the Falchion into the air and prepare for that long waited Critical Finish. A deep voice would yell "Dragon's Vein!" which causes the water in the stadium to wash Marth towards the side. Robin would lift himself off the ground, confused at what he saw and looked to see the soaked prince. Then young prince similar to Marth lands in front of him, with a mysterious flair and lack of shoes to show that he's out of the ordinary.
"Are you alright? You do know that you shouldn't underestimate the old generation," this prince said. Robin would take a look at the mysterious prince's blade Yato, which shines even brighter than the level sword. The soaked Marth would then stand I his two feet and prepare to fight again. The prince would then say "Dragon's Claw" for his left hand to transform into that of a dragon's to much of his surprise. Robin would then look at this Avatar and realize that he's going to be in a better situation than him. "Stand back, things are going to get tough, just like the fates of others that had to choose a path like mines," he said then he charges at Marth with his claws ready for tearing.
"Corrin Determines His Fate!"
*cue gameplay at various stages including the Nohrian stadium*
Again, that's all I have for now since I can seem to think of a good ending other than revealing Corrine/Corrette. Maybe have Marth complain about how stranger the Warriors are getting (as a shoutout to the hardcore fans that hate the new games) and have Robin watch him transform into his true self.