imo there's no way Hawkins is Killer's final fight in wano
He was originally going after big mom with Kid but Hawkins got in the way. I think after he beats Hawkins, he will offer to spare Hawkins's life only under the condition that he joins the Kid pirates. Hawkins will agree, and the two of them will intervene in the big mom fight and help Kid win.
Hawkins and Killer could honestly make a huge impact on the big mom fight. If Hawkins can make a voodoo doll of big mom, the supernova would easily be able to deal major damage to her.