Sanji and Zoro will be getting devil fruits soon
Oda has been foreshadowing these fruits for these powerhouses since Water 7. Zoro was shown to have a slight power of Asura in the fight against Kaku, obviously he didn’t become Asura because his current body doesn’t allow for the full transformation.
Sanji in his fight against Jabra unleashed his Diable Jambe, obviously a nod to the devil, also known as Diablo in hispanic culture. Same thing goes, his body could not handle the immense power of Diablo.
Oda is known to foreshadow very early in one piece so I believe that these two will be receiving devil fruits, probably during Wano.
The names of these fruits will be Asura Asura no mi and the Diablo Diablo no mi. They will mythical Zoan types. Both of these fruits allowing the users to completely transform into the demons. Their base powers will obviously both be upgraded since they will awaken their fruits full potential during Wano, since they have experience unlocking these powers without the fruit, along with being given immortality. Asura’s are known to have superhuman abilities, along with the extra limbs that Zoro is given in his initial activation of his Asura ability, so there will not be any special abilities besides the increased physical capabilities. There are many theories out there about Zoros eye, with the consumption of the Asura Asura no mi opening his eye will open a gate to the underworld where Zoros opponent will be sent only after they have been weakened a significant amount. Sanji will Receive a buff as well with the consumption of the Diablo Diablo no mi, his physical capabilities will not change as much as Zoro but he will have full control of fire. He will also be able to summon demons from the depths of hell and command them. Every step Sanji takes will melt everything that has the misfortune to touch his feet. (Fun fact: Sanji can also cook faster with this devil fruit)
It is Necessary that these two consume a devil fruit in order to compete in the New World. Luffy is far beyond their skill with his Gear 4th, especially when Gear 5th is on its way as is. Even with that Luffy cannot hope to be King of The Pirates without a solid foundation in his crew to help him fight the Yonko.
*Theory by MoogleComplex