@Bartholemew:
This definitely is an interesting question. Aside from which Yonkou will it be, I'm really sure that this saga will end with a Yonkou battle which at the end of the day will be a 1 vs. 1 between Luffy and the Yonkou.
What do you mean with "at the end of the day"? In a similar way as Luffy vs Doflamingo happened? Because I don't think Luffy on his own is ready to beat a Yonkou any time soon. He beat Doflamingo by the skin of his teeth and only after the latter had been quite severely damaged by Law already - not to mention all the teleportations Law did to help Luffy even at the end of the battle. So I don't think that he will beat a Yonkou proper "one on one" yet. Maybe with the second or perhaps even only the third Yonkou fight (assuming there will be that many; I sure hope so). If you meant "one on one" in the Luffy vs Doflamingo way then yes, that is a possibility.
I'm also not entirely too sure if it will be at the end of "this saga". Kind of depends on how long this saga will be I suppose. I'm not sure if I have a concrete basis upon which to base this assumption, but I always felt like Admirals were a bit below Yonkou level (I think because of the Whitebeard war), and that Doflamingo was pretty much at Admiral level. I feel like Luffy is ready to take down an Admiral very soon, and I kind of feel like this may happen before the first Yonkou falls, especially if my assumption about their general power hierarchy is correct. Ryokugyu will probably be introduced in his own arc soon, so it may be him. It may also be Kizaru (and Sentoumaru), since he was the one present when the Straw Hats suffered their pre-timeskip defeat. It would be fitting for those characters to go down by Luffy's hands some time.
All in all it's very much open to interpretation and we will really just have to wait and see, but the things I listed above make it questionable for me whether a Yonkou's downfall will happen by the end of this saga.
That said I still have the feeling that the Yonkou who will deliver this fight will be Kaido. We had so much foreshadowing (Punk Hazard, Dressrosa), not to forget the actual foreshadowing for Kaido started way back with Thriller Bark and showing us what kind of broken man Moria has become thanks to Kaido. We didn't have any real Big Mom foreshadowing pre-time skip other than that she exists, did we?
That is definitely true, I agree with you on that. Furthermore, Kin'emon has already traveled with the Straw Hats for a while now, I feel like it is time for the Wano arc to happen soon. And since it's been highly suggested (if not explicitly stated?) that Kaidou is in control of Wano, or at least that he has caused a stir in there, the Wano arc seems like it should be tied in with the Kaidou plot. I feel like Elbaf will tie in with Big Mom in the same way, mainly due to the fairy tale-ish theme that surrounds both that island and her crew. (Similarly to how the theme right now is animals/beasts as we are on Zou and Jack just caused a ruckus there.) Since Wano has been set up for so long now (not saying Elbaf has not, but I mean when looking at Kin'emon) I feel like we will go there before going to Elbaf.
Even this cannot really lead us to concrete conclusions, though, as who is to say that Kaidou will be fought in Wano, or Big Mom in Elbaf? What will be the setting where these people go down? Usually, Luffy has good reasons for wanting to fight people, and even though he already wants to beat all Yonkou, I feel like Wano and Elbaf will at least be stepping stones in the narrarives that lead to the downfall of Kaidou and Big Mom respectively. But whether they will be fought on those islands themselves remains to be seen. Perhaps the Straw Hats will infiltrate Whole Cake island for Big Mom eventually. Who knows?
leaving PH-
post battle-
[/hide]By the way, Kougetsu (光月) clan in chapter 816 means "Shining Moon" clan in Japanese. Maybe Wano has something to do with space…