This will most likely proven wrong next chapter but still
I think every devil fruit user can become Nika because Nika is origin of every single devil fruit.
Nika was like father of devil fruit or something.
Why someone with year- year fruit has a imagination powers? because of strong believe in Nika.
Bonney was already able to access Nika's powers
Next one to become Nika is probably Chopper.
He has a pink theme too
Side effect of smiles is uncontrollable laughing because they are clones of Devil fruit and every devil fruit is created from Nika
Luffy's devil fruit is still Gomu Gomu no mi.
He is just the first one to reach the highest level
Only one who can't become Nika is Blackbeard.
Blackbeard was looking for Yami Yami no mi because this is the only devil fruit that was not created from Nika.
We are probably getting different power up for Luffy and most of the devil fruit user will become Nika in the end.