I personally hope that the last leader we battle isn't Sapphira. I've always felt that, since most dragons are pseudos, it's one of the strongest types by default, if not the strongest; at least at face value. Seems kind of cliche to me to battle Sapphira last because of that. Also, there's no guaranteed that we fight Amaria in this Episode, despite the episode title; she and Titania have to fix their relationship first. Besides, Amaria's trainer class in the PBS is "Fiancee", which I don't think she is at the moment. Unless that's just a placeholder and will be changed in later episodes. Amaria's also my favorite gym leader, so I'd personally love to fight her last.
Furthermore, we know that one episode will have two leader battles, and that 2 leaders will be battled on the Dragon Field. If, after the desert, we learn that the missing twins found a way into Labradorra and we liberate that city before Agate, it would be super easy for us to battle Sapphira and Hardy in the former's gym back to back on the dragon field. We could the liberate Agate and battle Amaria afterwards.
My ordering: Titania -> Sapphira -> Hardy -> Amaria.