I don't think that a conservative will challenge Graham because of a dislike for his mentor McCain, I think it will be because of Graham (i) taking positions that are too liberal for South Carolina conservatives (which might be acceptable to them for an out-of-stater who is running for President but not for one of their Senators) and (ii) perhaps being, in the words of the SC Democrat Chairman, "light in the loafers."
I don't think someone from Up Country SC can or will challenge Graham, which is why Ravenel and Sanford make more sense to me.
As it was, South Carolina would (in the past) have a Senator from the Up Country and Low Country (Thurmond being from the "Up" and Hollings from "Low" -- although there was a 12-year overlap when Olin Johnston and his interim successor, Donald Russell, along with Thurmond represented the Up Country -- Thurmond's elected predecessor, Burnet Maybank was a "Low Country" Senator), so Ravenel could argue that his being Senator would more fairly geographically represent the state (as DeMint, too, is an Up Country Senator).