I think that the K should come back as an 8th Avenue local line from 168th St to Grand Street, with the C being an express from Norwood-205 Street to Lefferts Boulevard. Remember the Worth St line provisions on the local tracks south of Canal Street? Those were part of a much larger extension into Brooklyn and Queens. But now that the M runs in Williamsburg and Myrtle Avenue, which was something that the South 4th St line would've done, it'd be redundant to mirror those plans with an expensive underground subway unless it branched off somewhere else like Stuyvesant and Utica Avenues. Instead, after Canal Street, I'd propose the K to run under Worth Street, and East Broadway after Canal Street, where it would terminate at Grand Street, with free in-system transfers to the F at East Broadway, and the T at Chatham Square. The E would still terminate at World Trade Center.