Pretty obvious the other fruit Rocks wanted was the Yami. It'll be Davy Jones DF and the reason why Blackbeard specifically waited for it to start his plan.
To me this isn't necessarily obvious. BB and xebec most likely share the same overall goal but BB is definitely going about this his own way. Xebec was a supremely skilled martial artist. Meanwhile BB instead of following this path made a bet on getting strong devil fruit abilities with one specifically being meant to shut down enemy devil fruits. We got a pretty detailed look at xebec's life but not once in all of it we saw him express any amount of concern regarding shutting down enemy devil fruits. His interest in devil fruits seemed specifically related to the world history and davy jones. He didn't even want the nidhogg fruit for himself, he wanted it for harald since harald was the one who could bring out it's true potential.
Now, I do think the idea that xebec wanted davy jones fruit is probably at least likely. But it being the yami fruit is unlikely. Originally I was of the idea that maybe davy jones' fruit was imu's devil fruit but the more I thought about it the more awkward the idea seems. Suppose the devil fruit related to the promise between Imu and Davy... If the promise was to return the power to Davy or his descendants then that may required Imu dying since we don't know any other method to retrieve a devil fruit, let alone THE devil fruit. What I've come to believe is that the devil fruit Xebec is after is a new one that has not been introduced yet and it is currently in the possession of Imu at Mary Geoise. I think there are a few things which put together are very suggestive of this:
1.- Xebec was looking for 2 fruits. One of them is confirmed to be the nidhogg fruit.
2.- 50 years ago xebec broke into mary geoise, confronted imu, killed an admiral, escaped and fatefully met the ideal owner of the nidhogg fruit.
3.- Xebec then gathered the rock pirates, explained his goal and told them he was after two devil fruits.
4.- Xebec made multiple attempts to recruit harald through his life and established a rather awkward friendship with him.
5.- Xebec ultimately dies at the hands of garp and roger while he was domi reversi-ed without ever finding the fruit he was looking for.
6.- 10 years ago shanks stole the nika fruit from a world government ship. The WG's government's intent here is obvious, it's to prevent the fruit from circulating.
7.- BB actually explained to ace why he specifically wanted the yami fruit. he didn't mention itwas required to claim the world, he simply said it shut down enemy abilities.
8.- BB has also made a point of providing his executives with top rated abilities. If BB is after davy's fruit this is also insurance that they won't attempt to steal the fruit for themselves at a pivotal time. It keeps their interests aligned.
9.- in more recent times we see katarina devon obtaining saturn's image. What could BB want this for? IMO the most likely scenario is that he intends to have catarina break into mary geoise and roam around with some amount of freedom. It's likely that saturn never got around to telling people catarina stole his appearance and even with saturn being officially dead she could catch just about enough people off guard to get to where BB needs to get.
11.- Imu has a giant freezer where he keeps a giant straw hat. We only saw the strawhat but it's not necessarily the only thing there.
So, we have xebec breaking into mary geoise and BB most likely having an interest in doing so. We know the WG has been looking for devil fruits that are important to the past so it's not impossible it succeeded in getting at least one. BB has gathered a crew to whom he provided powerful abilities which not only strengthens them but also removes their incentive to take new fruits for themselves. Imu has a storeroom which devon may just have the means to reach.
Too add a final cherry on top of this, Mary geoise is currently under substantial turmoil. Not only saturn died and was replaced but MG is also under siege which is keeping the god knights and gorosei preoccupied. Not only that but things are dire enough that this is likely an all hands on deck situation so even high ranking marines stationed there are most likely involved in the defense as well. AND finally, imu left MG. To a place where time is weird for some reason. IF the fruit xebec was looking for is at MG then this is absolutely, definitively the best possible time to retrieve it. It wouldn't be weird at all if when imu returns to MG the BB pirates are long gone with xebec's prize.
