The only people I can imagine actually giving Feinstein a run for the money are:
Kamala Harris (already Senator)
Ro Khanna (just beat Mike Honda for a House Seat)
Hilda Solis (not going for it and would be a YUGE dark-horse)
And besides Harris (who is already Senator), none of them would really come all that close...probably about the same margin as Kentucky 2014 if lucky.
Khanna and Solis won't run. None of the serious Democrats want to burn bridges in the party by running against Feinstein. Would Eisen win a few counties, or would Feinstein win them all?
Already alluded to that.
And as for Feinstein sweeping all the counties, that's a YUGE possibility, especially just because of the anti-Berkeley bias in some of those Inland and Southern Counties that otherwise might be sympathetic to anti-Feinstein sentiment.
I'd guess Alpine or Mono counties would be the biggest resistance areas in such a case, unless Eisen were to somehow be able to ride as the "anti-Establishment" or "Feinstein's too old" camp.