No, "the feds" aren't discussing it. Read the whole article in the OP, not just the headline.
The head of DHS was asked if they were considering arresting sanctuary city leaders, and she responded "We're looking into all avenues".
That's a non-answer to a reporter, not a statement that it's actually being discussed.
But that's beside the point. The SCOTUS has also ruled, numerous times, that local law enforcement is not subordinate to federal law enforcement.
If ICE asks the SFPD to hold someone, without a warrant, the SFPD can tell them to go **** themselves.