Wait, he was Red Sox outfielder?
Rickey Henderson (2002), Kevin Mitchell (1996), Grady Sizemore (2014)
This hypothetical team really hits the jackpot with the outfield with two former MVPs and, when healthy, a perennial All Star. All three of these guys played all over the outfield for the Red Sox, so it was easier to just give the three blanket outfield spots to give this trio its time in the sun.
Rickey Henderson had the best career of anyone on this team, and is, in my mind, one of the most underrated legends of baseball. The man could just do it all, arguably deserved more than just one MVP award, and his stolen bases record will most likely never be beat. He spent 72 games with the Red Sox at age 43, and while he wasn't near the player he once was, it was still exciting to see Rickey Henderson representing the Red Sox.
Kevin Mitchell, despite his injury troubles, had a very strong peak with the Giants in the late 1980s, culminating in winning MVP in 1989. He made his way to the Red Sox in 1996 for 27 games, before being traded to Cincinnati as his last attempts to end his career on a positive note marched on. While Mitchell was solid for the Red Sox (.304 average, 102 OPS+), his injuries hampered what could have been a helpful piece to the Red Sox.
I remember being more excited for Grady Sizemore joining the Red Sox than I should have been. I was extremely hopeful that the Grady Sizemore that only missed 5 games in 4 seasons from 2005 to 2008 for Cleveland would show up for the Red Sox, but we got another case of the injury bug leading to Sizemore being designated for assignment after just 52 games with the Red Sox. Sizemore is a great case of a "what could have been?" career, and I wish the Sox could have gotten a little bit of Sizemore's star ability.
Honorable Mentions: I would like to take this space to sincerely apologize to Mike Cameron and Scott Podsednik for not including them in the outfield for this team, but the star power of Henderson, Mitchell, and Sizemore was too good to break up.