Free Agent to let walk: 1B Carlos Santana
This may be somewhat tough as the Brewers just brought Carlos Santana in at the Trade Deadline and he's performed just about as expected. He's added some power to Milwaukee's lineup, slugging seven homers since his arrival, and hitting for around a .230 average, which is what can be expected of him at this point.
Santana's biggest value is his veteran experience and leadership, which will no doubt be of use to Milwaukee in the playoffs this year.
The 37 year old Santana is a free agent at the end of the year and it doesn't make much sense for the Brewers to sign him back for another season. Milwaukee has other options they could go with at first base and DH that are younger and will be cheaper as well. Santana is making $6.725MM this season.
The Brewers' MO generally doesn't involve signing veteran position players in their upper 30s to full season deals. They'll bring them in at mid-season if needed as reinforcements but it's hard to envision them signing Santana back this offseason.
Right now the Brewers are in the process of transitioning cores. They're remaining competitive while several key players, Corbin Burnes, Brandon Woodruff, and Willy Adames among them, are nearing free agency themselves. There's been a youth movement this year with several rookies and there will be more in the future. Milwaukee let veterans walk last year to clear room for these rookies and they aren't likely to keep Santana around to potentially block playing time for the next wave of rookies.
There's a lot to like about what Santana brings, but the Brewers need to let him walk this offseason.