Rex, with Pyra and Mythra (think Pokemon Trainer) as a unique makes way too much sense, especially considering the game success.
And for Echo: DARK BOWSER!!!! Make it real, Sakurai
My (locked) thread about how difficulty should be a decision for the developers, not the gamers.
https://gamrconnect.vgchartz.com/thread.php?id=241866&page=1