Let's break it down a tad. Keep in mind this is all from memory and purely speculative in nature.
Available Candidates-
Titania, Amaria, Ciel, Hardy, Sapphira, Adrienn (don't blame me if mispelled. I used the OP spellings)
Titania- Steel: Looks like a tough battle because steel is a strong and versatile type. I don't think this one is plausible because we haven't seen any new field effects that would give her the edge other than the Mountain field. It is quite possible I am wrong, though. We might just end up in the factory field again.
Amaria- Water: This one looks quite promising with the host of new water fields added. Definitely possible.
Ciel- Flying: I don't expect this one to come up soon due to the fact that we just beat Terra and I don't feel like Ame would let us stay in the same location. Also, I expect we'll receive the TM Acrobatics for her defeat, which is a rather strong move all things considered. I don't think this is likely.
Hardy- Rock: Hardy, as a Reborn League Leader and a player, worked to prove the Rock type was a force to be reckoned with. I don't think we'll be facing him since 1) he was practically the pinnacle of the skill of the league and 2) he's too concerned with Aya's capture to be thinking about gym battles at the moment, not to mention he has his own gym in Agate City and a few of the gym leaders we face don't really have their own "gyms" per say, as in actual buildings with the one and only purpose of being a place for up and coming trainers to challenge one of the best in the region. Many of the leaders we face have "gyms" as in impromptu stages which will serve as their gym during their slightly askew situation, such as Noel, Julia (remember, her gym was a power plant first and thus it isn't really a "gym" as I previously defined), Samson/Ciel, Terra, and Luna. Hardy does actually have his own gym gym, which is simply a gym and not for any other purpose (that we know of) so unless his gym is actually destroyed by the Meteors, I don't see this happening.
Saphria- Dragon: Dragon is designed to be an insane type to fight. It resists every starter main type and murders most types in return. That being said, Dragon is often used as a final challenge to test the true perseverance of the Player. A couple good examples of this are in BW with the two Dragon leaders which gave out the Eighth, and final, Badge and the infamous Dragon Tamer of the first four generations: Lance. I suspect Saphria, the dragon leader constantly shown to be insanely powerful with her ability to wipe out a lake full of Meteors in the time it takes you to fight a battle or two and also later tames an enormous Steelix because Wynaut? I feel, based on these facts, she will be leader number #18 and be the final bar from the Pokemon League.
Adrienne- Fairy:(I apologize in advance if I accidentally screw up the non-binary pronouns. I am a tad rusty) Xe's a real possibility. It seems like we're heading back to the city soon enough and we'll be able to see xyr good work. Not to mention, xe already has a neat field effect implemented in the Coral Ward.
That's my analysis. Keep in mind, I haven't played through it in a while and it is all from memory. Feel free to add to or refute.