One way that Machado may still be improving

It feels a little odd that we may need to point this out in the midst of one of the most-examined free-agent cases in recent memory, but it seems like maybe we do: Manny Machado enters the market having had his best hitting season. That's not faint praise, because his first five seasons all ranged from good to great. Machado's 2018 was better than any of them, by almost any measure. It's incredibly impressive.
Beyond that, Machado has hit the ball hard more often than anyone else in the four years of Statcast™ tracking. He's actually in the midst of a hard-hit trend that only three other players can match, for what it's worth. Machado's hard-hit rate has increased every single year. 
2015: 41.9 percent of batted balls 
2016: 43.6 percent
2017: 47.7 percent
2018: 48.2 percent

We define a "hard-hit ball" as one that's struck with a 95 mph exit velocity or more, and it's just about the best thing you can do as a hitter. (This should be intuitive -- of course it's good to hit the ball hard!)
In 2018, hard-hit balls across the Majors had huge numbers: they produced a .524 average, a 1.048 slugging percentage and nearly 97 percent of home runs. Machado's numbers were similar: a .559 average and a 1.118 slugging perecntage. 
Unsurprisingly, balls hit 94 mph or less didn't have nearly the same production, coming out to just a .219 average and a .259 slugging percentage. 
It's why names like Aaron Judge, Christian Yelich and J.D. Martinez can be found toward the top of the hard-hit leaderboard, and Delino DeShields, Dee Gordon and Billy Hamilton are toward the bottom. Hitting hard isn't the only thing, but it's incredibly difficult to succeed without doing it at least a little.
That being the case, showing improvement in that metric year over year can only be considered a good thing. When we dug into it, we found 10 other hitters who recorded at least 200 batted balls in each of the past four seasons and also improved their hard-hit rate each season, but it's also not exactly a fair comparison. It's technically true that the light-hitting Alcides Escobar improved each season, but he also started at a terribly weak 16.5 percent in 2015 (among the lowest in the game that year) and made it up to only a similarly weak 21.5 percent in 2018.
That's basically half of what Machado's lowest number was, and that's not exactly what we're looking for. Instead, let's restrict it to those who improved each year and started out with at least a league-average rate, which in 2015 was 33.5 percent.
We're left with Machado and three others. Interestingly, all four of these players might be expected to find new homes this winter.
Four years of improved hard-hit rate in 2015-18
Machado (from 41 percent to 48)
J.T. Realmuto (34 percent to 41 percent)
Marwin Gonzalez (34 percent to 41 percent)
Avisaíl García (38 percent to 43 percent)
Minimum 200 batted balls in each year, and a league-average starting point in 2015
That's it, and it's something of a fascinating list. Realmuto is baseball's best catcher who is obviously in high demand as a trade piece this offseason and Gonzalez is going to be a valued free agent for his bat as well as his versatility. The story isn't quite so rosy with Garcia, who was non-tendered by the White Sox last week, though that has more to do with a massive increase in strikeout rate than anything else.

In Machado's case, this trend is the continuation of a theme.
He led the Majors in hard-hit balls in 2018 with 257 (28 ahead of second place).
He led the Majors in hard-hit balls in 2017 with 250 (20 ahead of second place).
Since Machado was seventh in 2016 and sixth in '15, then you can add up the numbers and guess where this is going next: no one has hit more hard-hit balls than Machado has since '15, by quite a large margin. 
Most hard-hit balls in 2015-18
957 -- Machado
866 -- Mookie Betts
843 -- Nelson Cruz
841 -- Robinson Canó
835 -- Yelich
It's fair to note that the durable Machado has also taken the most plate appearances over the last four years (2,808) though he still rates well on a hard-hit-per-swing basis.

Now, all those hard-hit balls only matter if they turn into production, and for Machado, they just did. His 2018 line of .297/.367/.538 breaks down like this:
On-base percentage: .367 (career best)
Slugging percentage: .538 (career best)
Strikeout rate: 14.7 percent, lowest (career best)
Walk rate: 9.9 percent, (career best, ever so slightly ahead of 2015)
Home runs: 37, (career best, tied with 2016)
Weighted on-base average [wOBA]: .377 (career best) 
Weighted Runs Created Plus [wRC+]: 141 (career best)
Hard-hit rate: 48.2 percent (career best, obviously)
Avg. exit velocity: 91.6 MPH (career best)
If you care about batting average and RBIs, which you probably shouldn't, Machado's .297 and 107 were also his best marks. 
It's difficult, in some sense, to look at "what changed," because it's not like Machado is a Max Muncy-esque out-of-nowhere story. He's had his ups (2015, '16 and '18) and downs ('13, '17), but he's of course been a star-level performer since basically Day 1. We can, however, take note of a few trends.
For example, pitchers seem to fear Machado more, throwing him more and more pitches outside the zone each year, while at the same time his barrel rate continues to improve. 

All of which is to say, when it comes to the kinds of trends teams will be looking at, Machado's are very much going in the right direction, and the top-level stats showed that he did indeed just have his best hitting year.
But if it doesn't feel like that this offseason, and it seems like maybe it doesn't, well, you already know why. Much of this offseason's discussion, aside from the third base/shortstop defense question, has been about the controversy generated over Machado's "Johnny Hustle" comment and when Christian Yelich called him a "dirty player" after he apparently spiked Jesús Aguilar in Game 4 of the National League Championship Series.
Those were self-inflicted wounds, so we're not skipping past them. They deserve the attention they get. You just can't focus on them to the extent that we forget that Machado's 2018 was actually something special. You can't ignore that in some important ways, he's still improving. Maybe that shouldn't be a surprise; he just turned 26 in in July. There's still so much here for Machado to offer, perhaps even the MVP-caliber season he hasn't quite put together yet. It's what teams are banking on. It's what's going to get him the massive contract he'll soon earn.