Minor chords sound like a better fit, but I'm not sure that high tempo chords are the best approach, unless, like you suggest, you simplify them a bit into the background and look for the End Title vibe, which sounds like an interesting option. I like this piece as inspiration for the second part as well because of its use of longer, sustained notes, which I think could really help to slow down Aerith's theme a bit, giving you some creative room to alter the upbeat feeling of the melody. If you slow it down it leaves you some room to more effectively change the feeling by adding an extra note to each segment of the theme to change it's feel, or to add a secondary melody after each segment as you did to great effect in the opening of the song. I apologize if my suggestions are unclear, unfortunately I lack the musical education to comment using more appropriate jargon.