I think another legendary that could potentially be released stat-wise would be Regigigas but lorewise you would need the other Regis to summon it, which are decent legendaries, so Regigigas would be out just because of that
Other than that Manaphy would be the next option due to how Phione is obtained in the games by breeding Manaphy but I'd say its too powerful for the current game, maybe Ame will make Phione found in the wild since if it could be obtained by breeding Manaphy it wouldn't be marked as found in the wild/repeatable event but rather it would be marked as obtained through evolution/breeding