Elio's endgame development arc legendary would still be either Zekrom or B-Kyurem; the two are one and the same. They both represent Ideals and looking toward the future.

Elio's entire character was reaching his ideal self through the culmination of experiences gained over the course of his life while reinforcing his personal belief that no dream or goal is too big - even if it can't be achieved in one lifetime, so long as you work towards it. Not to mention that it's important to live out your imagination - which can take you anywhere, stimulates progress, and gives rise to a new evolution, rather than your history - which limits your perception, stagnates growth, and leads you down a single path.