#4 Dylan Carlson
The switch-hitting Dylan Carlson is only two years' removed from being a top-10 prospect in the MLB. In his first full-season, Carlson hit .266 with a .780 OPS, good for a third-place finish in NL rookie of the year voting.
Since then, though, it's been a bit of a struggle. He's hitting .230 with a .680 OPS over the last two seasons.
He can play all three positions in the outfield, and while Dana Brown said the Astros aren't looking for any additional outfielders, the switch-hitter is a prime buy-low candidate. In his career, Carlson holds an .848 OPS against lefties. An outfield platoon of McCormick in left, Carlson in center and Tucker in right with Yordan as the DH would give left-handed pitchers fits.
Carlson is a great base-runner, above average defender, and he is a patient hitter that doesn't expand the zone or whiff. His approach would fit in seamlessly in Houston.
The Cardinals have a logjam of outfielders, and have been forced to play Jordan Walker there out of position. If they do trade from their surplus, the Astros could trade from their starting pitching surplus and make a good match.