Most people assume Rocks wanted to gather two specific devil fruits before confronting Imu. One is confirmed to be the Ryu Ryu no Mi, Model: Nidhöggr. The other one is supposed to be the Hito Hito no Mi, Model: Nika.
This last part is, in n my opinion, wrong.
We know from what BB said in the past that he had a book which gave information about all the devil fruits ever recorded. At the same time we know that the Nika fruit was recorded as the Gomu Gomu no Mi, probably because the truth about the fruit was obscured on purpose. The World Government had tried to capture the Nika fruit for 800 years but always failed to do so (they almost had it but Shanks took it from them, before Luffy ate it).
So what are the chances BB knows the truth about Gomu Gomu no Mi? Not high imo, especially because BB met with Luffy and never went out of his way to steal his devil fruit. It is therefore likely BB is not informed about Nika, nor were Rocks and his mother.
So, if the second fruit BB is after is not Nika’s what is it? The answer was probably given by the latest spoilers. It is the rain goddess’ fruit, Zaza and the one holding the fruit is none other than the world’s most wanted man, Dragon.
This reasoning is based on two elements: one is in the latest spoilers where it’s mentioned Zaza is the most feared goddess in the Holy Land. Why fear her so much if she wasn’t the key to defeat Imu who is the pillar on which the whole celestial dragon’s legitimacy rests?
The other element comes from the story thus far. Unlike with Luffy, BB went after Dragon and the revolutionaries HARD. So much so he forced them to evacuate Baltigo. BB has always been presented as someone who is very deliberate and consistent with his plans. Someone who never takes unnecessary risks and only acts when he thinks he’s set the board in his favor.
Why then go against the revolutionary army and Dragon? These are dangerous and powerful people who do not bother pirates. It makes no sense for BB to confront them… UNLESS they have something he desperately wants.
That thing in my opinion is the rain goddess devil fruit, which is the second devil fruit Rocks needed to defeat Imu but could not find in time because he was forced to confront it before he was ready. Those same devil fruits are also what BB is searching for, which will put him and his crew on collision course with the Straw Hats!