Since 1901, Fielder is tied for the 35th-best OPS+ over the 3 seasons comprising Age 25, 26, & 27 (minimum 1500 plate appearances.)
What does that mean for the rest of his career? Click through for more.
Here are the top OPS+ values from the above list, just since 1960:
There's Fielder at #12, tied with 3 Hall of Famers in Eddie Murray, George Brett, and Carl Yastrzemski. Murray finished with a career OPS+ of 129. Brett had 135, and Yaz had 129. It seems like a reasonable guess that Fielder will finish in that range as well.
Following his Age 27 season, Murray posted 8,362 plate appearances with a 122 OPS+. Brett had 7,294 with a 133 OPS+. Yaz had 9,384 with a 127 OPS+. With Fielder, I'm of two minds--of one hand, he's a tough SOB who has played in a ton of games. On the other hand, he's a heavy guy and seems destined to have knee or back problems eventually.
So let's just take our best guess (an average of the 3 HOFers) and suppose that he finishes his career with 8,347 additional plate appearances and a 127 OPS+. What does Fielder look like with a 127 OPS+? Well in 2008, he had a 130 OPS+, and 0.049 HR and 0.147 RBI per plate appearance. That was considered a worrisome year at the time, but it would be a great level for him to perform over the rest of his career.
If he maintains those above rates over another 8,347 plate appearances, that will result in another 409 HR and 1,227 RBI. Adding to his current totals, that would put him at 639 HR and 1,883 RBI. Were he to achieve that, he'd pass Ken Griffey Jr for 5th in homers and Mel Ott for 12th in RBI.
That's a nice thought for whichever team lands him as a free agent, but of course he's only about a third of the way there...