Luffy literally freed Yamato by destroying her physical chains and also freed her from her father by defeating her idols killer, destroying her psychological chains at the same time. That's an incredibly deep connection to Ace because Luffy did the same for him. He freed him from his psychological chains that troubled him due to being hated for being Rogers son. He freed him from his father similar to Yamato by accepting him just like he is. And in MF he also freed him from his physical chains because without Luffy, Ace would've died right there on the scaffold. That in itself is one of the best subtle plots that Oda included in the last Arc or even the whole series.
Yamato will join, that's as clear as day imo. Otherwise Oda wouldn't have gone to such lengths just to intertwine those three characters in such a deep manner.