Steve Israel or Kathy Hochul are the most probable.
Israel's retiring. Hochul isn't a bad choice but she'll be easily tied down to Cuomo. Schumer at least has some liberal street cred, while Cuomo has none. (NOTE: Used the word liberal, not progressive, so before the left-wing gets triggered, calm down)
Gillibrand was going to be but then she started doing her own thing in 2013.
Kathleen Rice seems to vote virtually like he does. Grace Meng not far off, either. Both voted against the Iran Deal, tow a semi-hawkish line on foreign affairs, and Rice supports TPP (Meng was against it).