There's more than one way to build a contending team, obviously. Just look at how the four clubs that reached the League Championship Series this year constructed their rosters.
They can all point to successes in the Draft, both high picks (like first-rounders Walker Buehler and Andrew Benintendi) and low (11th-rounder Joc Pederson or 33rd-rounder Tyler White). They've hit it big by spending in free agency, like with J.D. Martinez or Lorenzo Cain. They all have plenty of examples of little-noticed minor moves that ended up producing surprising breakout stars, like Jesus Aguilar, Justin Turner or Player Page for Max Muncy, and they've all found gems on the international market, either professionally (like Hyun-Jin Ryu and Kenta Maeda) or as amateurs (like Xander Bogaerts, Jose Altuve and Yasiel Puig).
The point is that there's no one right way to do this. You have to add talent any way you can, and the best teams take those players and make them better. But there is one common thread running through these four clubs, one that could inform you about what to expect in the upcoming Hot Stove season. Each of them added about a third of their team value this year via trade. Each of them has found more value in trade than they have in free agency. 
Think about the names we're seeing on the final four clubs acquired via trade. When we point out that huge names Justin Verlander, Chris Sale, Christian Yelich, Gerrit Cole, Josh Hader, Craig Kimbrel, Rick Porcello, Manny Machado, Chris Taylor, Travis Shaw, Corey Knebel, Rich Hill and Mike Moustakas all initially landed with their current club via trade, we're just scratching the surface -- there's also Thomas Pressly, Joe Kelly, Enrique Hernandez, Brock Holt, Steve Pearce and so many others.
In fact, if you look at breakdowns of the different avenues of how each team acquired production this year (expressed in Wins Above Replacement), you can see that for each of our final four teams -- and Major League Baseball as a sport -- the largest share of 2018 production was procured via trade.

Obviously, free agency still has a big role. Cain and Martinez are probably the two largest success stories of teams being aggressive and spending for free agents from last offseason (as we attempted to point out at the time), and the final four have also benefited from Jhoulys Chacin, David Price, Mitch Moreland, Josh Reddick, Joe Smith, Hector Rondon and others. This offseason, Machado, Bryce Harper and Patrick Corbin are going to get enormous contracts -- and they're going to deserve them.
But the once-historic 2018-19 free-agent class no longer looks quite so dynamic, thanks to age and injury. When we look back at last offseason, some of the biggest moves ended up coming via trades -- Yelich and Cole, but also Giancarlo Stanton, Marcell Ozuna, Andrew McCutchen, Stephen Piscotty, Matt Kemp, Corey Dickerson, Joey Wendle and Dee Gordon. At least part of the reason is access to younger stars, since baseball has been trending towards youth for years.
All of which is to say that while we focus on Harper and Machado this offseason, the most interesting action might not be in who signs McCutchen, Andrew Miller, Zach Britton or Adam Jones. It might be in who makes the best trade, like Milwaukee did for Yelich. Here are 15 potential names to keep in mind.
C J.T. Realmuto, Marlins
Ever since the Marlins tore it down last offseason by trading Stanton, Yelich, Ozuna and Gordon, the question has been whether they'd continue by trading Realmuto (.277/.340/.484, 21 home runs), who has two seasons left before free agency and is probably the best all-around catcher in the game. It would be nice if Miami could build around him, but if it can't sign him to an extension, we're going to hear his name in many rumors.

Working in the Marlins' favor is that the state of catching in the game right now is weak, with many clubs needing a boost, and likely top free-agent catcher Yasmani Grandal has done himself no favors with his high-profile struggles in the postseason. Wilson Ramos and Jonathan Lucroy are available as well, but neither is of Realmuto's caliber.
Possible fits: Rays, Red Sox, Nationals, Astros, Rockies, Brewers, Braves
SP Madison Bumgarner, Giants
This can't happen until the Giants name a new general manager, and in some ways it's difficult to envision the new hire arriving and immediately trying to move such an iconic player. Then again, the reason San Francisco even needs a new GM in the first place is that it has finished last and fourth in the past two years. Since the 2016 All-Star break, only the Orioles and White Sox have fewer wins. Change is coming -- and Bumgarner becomes a free agent after 2019.

Possible fits: Yankees, Braves, Angels, Astros, Brewers, Nationals
3B Nolan Arenado, Rockies
We'll admit that the Rockies don't seem likely to want to move Arenado, especially since their weak offense was already an issue even with him, and their main goal ought to be a long-term extension to keep him in Denver alongside Charlie Blackmon, Trevor Story, and a good young starting rotation for many years to come. That said, he's entering his final year before free agency, and if they can't reach an agreement, they might not want to simply watch him leave next offseason -- and young Ryan McMahon is ready now.
Possible fits: Phillies, Braves, Indians (if Jose Ramirez plays second), Cardinals, Angels

1B Paul Goldschmidt, D-Backs
1B Jose Abreu, White Sox
1B Brandon Belt, Giants
1B/RF Jose Martinez, Cardinals
1B Carlos Santana/Justin Bour, Phillies
Let's toss in all of these first basemen together, for one very big reason: the list of free-agent first basemen this offseason is grim. Pearce, Lucas Duda and Matt Adams have their uses, but there might not be a single starting quality first baseman out there. If you want one, you'll need to get one via trade.
In some cases, these are strong hitters in their 30s entering the final years of their contracts (Goldschmidt, Abreu). In others, it's the Phillies desperately needing to move one or both of Santana or Bour to allow Rhys Hoskins to come back to first from the outfield, improving the defense. The Cardinals would do well to find an American League home for the defensively challenged Martinez, while the Giants, if they decide to move Bumgarner, might just want to do it all and trade the constantly underrated Belt too.
Possible fits: Red Sox, Angels, Mariners, Yankees, Twins, Rockies
LF Kyle Schwarber, Cubs
Yes, Schwarber will forever be a legend in Chicago because of what he did in the 2016 World Series, but there was an argument to trade him that offseason, and there remains one today, especially since the firing of hitting coach Chili Davis seems unlikely to be the only change made to what ended up being an unsatisfying offense.

While Schwarber did improve his defense somewhat, he's turned out to be more of a good player than a great one, with large career platoon splits (.349 OBP/.509 SLG vs righties, just .300 OBP/.308 SLG vs lefties) and probably better suited to some time at DH in the AL -- especially if the Cubs can get pitching in return. He still has three more years left before free agency.
Possible fits: Astros, Rays, Twins, Mariners (if free agent Nelson Cruz departs)
RF Nicholas Castellanos, Tigers
Speaking of corner outfielders who rely far more on slugging value than defense, Castellanos has one more year before free agency, and he just hit .298/.354/.500 with 23 home runs for Detroit. (Along with negative-25 Outs Above Average, the lowest mark in the game.) The rebuilding Tigers are unlikely to contend in his final year, and he would also fit best on an AL team looking to add some slugging and has DH time available. 
Possible fits: Astros, Rays, Twins, Mariners
2B Scooter Gennett, Reds
2B/OF Whit Merrifield, Royals
2B Cesar Hernandez, Phillies
Need a second baseman? There are actually decent options in the free-agent market -- DJ LeMahieu, Jed Lowrie, James Dozier or Daniel Murphy -- but teams might also be interested in this younger trio, who are team-controlled for one (Gennett), two (Hernandez), or four (Merrifield) years. 

If it's power you want, Gennett turned himself from a light-hitting Brewer into a slugging Red, putting up a line of .303/.351/.508 and 50 home runs over the past two years. Merrifield doesn't have that power, but he's stolen 79 bases in 2017-18 and can also play the outfield, to go with his .296/.347/.449 line. Hernandez has spent the past three years as a league-average bat, setting a career-high with 15 homers in 2018, and the Phillies might be motivated to make room to get Scott Kingery back to his natural second-base position.
Possible fits: Dodgers, Indians, Nationals, Red Sox, Twins, Rockies

SP Jon Gray, Rockies
Finally, a change-of-scenery starter. Gray looked like he'd be the next Rockies ace, but he's clearly been surpassed by Kyle Freeland and German Marquez, and he didn't even make Colorado's National League Division Series roster. It's not even about Coors Field in Gray's case, because his career splits are about even. It's that he's been so up and down in his short career that he was even sent to the Minors last summer, yet still had a strikeout rate (24.6 percent) as high as Noah Syndergaard or Clayton Kershaw.
Gray wouldn't come cheaply, because he's still young, talented and would come with three more years of control before free agency. Then again, this could be a good avenue for the Rockies to add the offense or relief pitching they badly need, and a team like the Astros might do wonders to bring out the best in Gray.
Possible fits: Astros, Twins, A's, Brewers, Cubs, Nationals, Mariners