Realism is important. You aren't going to get a high-end prospect for a rental, but you might as well get something. The players under team control consist of guys they'd be selling low on (Lindor, Alvarez, Baty), guys who are being paid more than they're worth (Polanco, Williams), guys who might opt out (Bichette), guys who will opt out but aren't healthy (Holmes), and one sell-high opportunity (Weaver). So I wouldn't expect anything eye-popping.
Making the team worse to placate somebody who will opt out at his first chance anyway makes no sense. I don't see it happening.
Alvarez has struggled to stay healthy, and it's hurt his performance both with the bat and the glove, but I think shipping him out under the assumption that improving on him will be easy would be a huge mistake.
Weaver would be far easier to replace than either Lindor or Alvarez.
Baty... the irony here is the Mets have held on to Baty when they presumably could have sold very high on him at a few trade deadlines or in offseasons, and now they'd be selling painfully low. Like with Vientos, I wouldn't cut bait just to be rid of him. I do think Baty has been playing hurt. But his upside, barring a substantially improved reading of the strike zone, is a useful reserve.