In this theory I will discuss the possibility that Nami will inherit the Soru Soru no Mi and how much of a power boost that will give her.
Also the assumption I am making in this theory is that Big Mom is dead right now.
So now to begin this theory:
1- Why Nami needs a Devil Fruit?
Let’s face the facts, as she is now, she is the weakest member of the Straw Hats. She can defeat fodders using her lightning and wind attacks. But these attacks can at most defeat only people of the level of rear admirals or really weak vice admirals.
Yes in 1v1 battles, she beat Khalifa and Miss Double finger, but that was mostly because those two characters got cocky and underestimated her. It was not because Nami truly overpowered them.
Post Timeskip she has not had a proper 1v1 fight. The closest to a 1v1 she had post timeskip was against Brulee and she struggled to beat her. And we are talking about Brulee who for most part is slightly stronger than a fodder character.
So if she has to beat someone like Catarina Devon or Laffitte( I feel Nami will battle one of these two when the Straw Hats take on Blackbeard Pirates because 1 is a woman and the other is the navigator), she has no chance what so ever.
So how can Nami become stronger?
First way is Haki, but to learn Haki one needs time and training. Or one must be born with special Haki skills like Ussop. Since Oda has already done that to Ussop I don’t think he will do it Nami. So, unless there is timeskip for Nami to properly learn Haki I don’t think Oda will go this way for her.
Second way is the Devil Fruit. Now she can eat a number of devil fruits like the Yuki Yuki no Mi of Monet or a devil fruit relating to weather, like a weather based Paramecia or a wind based Logia. Even though these are possible, I feel Oda will not take that route. This is because I feel Monet still has something left to do in the story and for the other devil fruits, the characters with these devil fruits have not been introduced yet.
There really would be no time to develop the new character and make Nami to get that characters power. And of course if Nami finds a random devil fruit she would rather sell it and make a ton of money than eat it and lose her ability to swim. So I feel the only way Nami will get Devil Fruit powers is if it is forced onto her or she chooses to inherit that person’s power.
But when it comes to the Soru Soru no Mi I feel Nami will either choose to inherit the fruit and or the power will be given to Nami accidentally from a certain someone when Big Mom dies.
Because of this I feel Nami will inherit a devil fruit and in turn use it to power up. Also since devil fruit usage does not need the same amount of training as Haki, she will get stronger pretty quickly.
Now these might not be the only ways she might power up and there could multiple ways for her to power up but since Oda has not introduced them yet, I am not considering them in this theory.