I expect them both to continue improving their swing decisions. As they do that, their OBP and slugging will increase. I also believe Hays was miscast as a lead off hitter out of necessity. I think it took away some of his aggression, but perhaps it was good for his growth in plate discipline.
I expect Hays to settle on his set up and the mechanics of his swing. He constantly changes things. In 2019 Spring training, before his hand injury, he was swinging the bat at an elite level with his hands a little higher and back near his load position. In 2020, he changed his hands set up to lower and hands more in front, causing him to be late often and without as much power.
I expect Hays to play LF as much as he plays CF, with an occasional DH, which hopefully helps him stay healthy. I expect Santander to play primarily in RF, with some DH.
Numbers wise, I think both will improve. Santander should be in the .280/.350/.550/.900 range. Hays .270/.340/.500/.840. Hays has every tool and can be a star, but his all out nature as an OF is a blessing and a curse. He has to understand situations and the importance of saving that for when it really matters.
Santander: .265 batting average, .325 on base percentage, 25-30 homers, 90-110 RBI.
Hays: .270 batting average, .340 on base percentage, 18-22 homers, good defense.
Hays staying healthy is the most important thing, though.
Expectations and projections aren't worth the paper they're printed on. He could do what we say or he could play in 20 games and miss the season with injury or hit .240 with 18 homers and 60 RBI. IF he stays healthy, I think our numbers are realistic. Expect and hope are two different things but I wouldn't be surprised if he did 32/105.