Return to La Defense would be an awesome gig
No, It would be boring because it has been done before.
As much as I like PLD repeating a concert is not an option IMO, the return to the Eiffel Tower proves that (ignoring Salut Eiffel from 1989 here)
If it has to be Paris, why not the Arche de Triomphe? (Although still to small for a JMJ concert)
Personaly I would love to see the Avignon Skyline lit up by our friend, but a concert on the foot of a mountain somewhere would be nice too and use that mountain as a backdrop similar to Swatch the world.