Zoro and Sanji bough fought CP9 member with almost equal (whatever that number of their strenght was).
Is Zoro was to fight Mihawk, Sanji should also get to fight someone. One of these two should become the next Dark King rayleigh and I mean in strenght. Zoro seems to be the most fitting one, since he is second one to receive a bounty and is a supernova, and is Luffy's first mate.
One of these guys needs to have Haki. Luffy can't be the only one to be able to fight Logias in the future. The way Rayleigh fought with Kizaru was cool.
Kaku and Jyabura had 2200 and 2180 Douriki respectively. But that was pre-Zoan, at least on Kaku's part. Zoro will fight Mihawk, but not anytiome soon, his dream needs to be fulfilled in the New World. And yes, Zoro will be the next Dark King, being first mate and all. Over time we will probably get to know more of Gol D. Roger's crew, and see pre-Sanji's and pre-Nami's and such. And you're right, there needs to be more Haki if indeed it is as helpful as we think, and the reason Rayleigh could fight Kizaru. But Haki on a Kuja warrior level would be good enough