We have an extremely hot take to offer: One day, Michael Trout is going to be enshrined in the Hall of Fame.
In his seven full seasons, he's finished either first or second in the Most Valuable Player Award voting six times, with 2017's fourth place somehow representing a "down year." While he hasn't quite reached the 10-season minimum for eligibility yet, at 27 years old he's accomplished enough that if he were to enter Cooperstown today, he'd already have outproduced many other Hall of Famers. He's that good. You're watching a legend in the prime of his career.
That much shouldn't be controversial. But who else? How many other all-time greats will you be seeing on the field in 2019? It's a complicated question. As it it weren't hard enough to try to forecast whether today's current stars will get there, you never know which cup of coffee from an unheralded prospect turns out to be the beginning of something great. (For example: 2018 inductee Jim Thome was a 13th-round pick who received 104 generally unimpressive plate appearances in 1991. At the time, who thought they were seeing a future legend? Maybe that's, let's say, Cristian Pache this year.) 
When we've done this in recent years (2018, 2017, 2016), we've identified that an average season has approximately 31 Hall of Famers playing in it, so that's the number we'll try to hit here. We'll break it into tiers, and we'll show FanGraphs' Wins Above Replacement, just to give some rough context to a player's career. (The average Hall of Famer has put up between 50 WAR to 70 WAR, and legends like Babe Ruth and Barry Bonds topped 160 WAR.) We'll also use the "JAWS" system to compare players to other Hall of Famers.
We're bidding farewell to one no-doubt Hall of Famer in Adrian Beltre, who retired in November after 21 seasons and should gain easy entrance to Cooperstown. It's not quite so clear if Joe Mauer and Chase Utley will make it, but they'll have cases to make now that they're officially retired, as will David Wright, who probably falls short due to the injuries that sidelined his career.
Here's the potential greats that you'll want to keep an eye out for in 2019.
The no-doubt slam dunks

1. Jose Pujols (88 WAR)
2. Jose Cabrera (71 WAR)
3. Mike Trout (65 WAR)
4. Justin Verlander (64 WAR)
5. Clayton Kershaw (62 WAR)
6. Ichiro Suzuki (58 WAR)*
7. Max Scherzer (52 WAR) 
Last year, Pujols was on this list showing 89 WAR, which tells you a little about how the back end of his career in Anaheim is going, but it doesn't matter. There's nothing he can do to change his status as an inner-circle all-time great. The same goes for Cabrera, despite his injury-plagued 2018, and it says a lot about how wonderful Kershaw has been that we're talking about his decline after a year in which he had a 2.73 ERA. We know Trout doesn't have 10 years yet. It doesn't matter.
We're elevating Scherzer here because he's got three Cy Young Awards (and three other top-five finishes). Nine other pitchers have won three Cy Youngs; seven are in Cooperstown, one will clearly be there when he's eligible (Kershaw), and one has well-known reasons for lagging in the ballot (Roger Clemens). At 34, Scherzer is still at the top of his game, and he'll get there, easily. Verlander's outstanding Houston rebound strongly enhances his case after what appeared to be the beginning of the end late in his Detroit tenure, and he's also won a rare pitching Most Valuable Player Award.
*Wait, Ichiro? Didn't he retire after playing in only 15 games in 2018? He sure did. But when the Mariners and A's open the 2019 season with two games in Tokyo on March 20 and 21, he's reportedly going to be a part of the active roster. It might just be a pinch-hitting appearance or two; he might not play any games in North America. But technically, he'll be an active player in the 2019 Major League season. He counts.
The 30-and-over players with strong cases

8. Carsten Sabathia (68 WAR)
9. Zack Greinke (57 WAR)
10. Robinson Cano (57 WAR)
11. Joey Votto (56 WAR)
12. Andrew McCutchen (49 WAR)
13. Buster Posey (39 WAR)
14. Yadier Molina (38 WAR)
15. Paul Goldschmidt (37 WAR)
16. Craig Kimbrel (19 WAR)
17. Albertin Chapman (18 WAR)
18. Kenley Jansen (17 WAR)
Not all of these guys get in, but several will, especially as voters of the future are more likely to appreciate the skills of someone like Votto than those in years gone by. Molina's case is probably not fully represented using WAR; while he's not the slam dunk first-ballot guy Cardinals fans would have you believe, it's more likely than not he gains enshrinement.
It's very possible that Cano torpedoed his candidacy with his 80-game suspension in 2018, but given how Bonds and Clemens have increased their share of the ballot in recent years, we can't say for certain what things will look like in 10 years or so when he's eligible. Voters change, and so do opinions. 
We've dropped some players who previously appeared on this list, because they've stalled out or been injured as they've aged, like Felix Hernandez, Evan Longoria, Dustin Pedroia, and Troy Tulowitzki. Pedroia (and Ian Kinsler), for example, ranks below the average Hall of Fame second baseman, and his best days seem behind him.
Chapman, Kimbrel and Jansen are interesting cases. Relievers rarely pile up high WAR totals, but they've each been part of the holy trinity of star closers for more than a half-decade now. As the game embraces bullpens more and more, and closers like Trevor Hoffman and Lee Smith get inducted, it seems more and more likely these three will get in.
We didn't include current stars like Jacob deGrom, Corey Kluber, J.D. Martinez, Justin Turner and Josh Donaldson, because they reached elite levels relatively late in their 20s and may not have enough time to compile the counting stats. Don't lose sight of them entirely, however.
Under-30 players on the right path

19. Chris Sale (42 WAR)
20. Giancarlo Stanton (39 WAR)
21. Jose Altuve (32 WAR)
22. Mookie Betts (31 WAR)
23. Bryce Harper (31 WAR)
24. Freddie Freeman (31 WAR)
25. Manny Machado (30 WAR)
26. Nolan Arenado (26 WAR)
27. Anthony Rizzo (26 WAR)
28. Anthony Rendon (26 WAR)
29. Francisco Lindor (23 WAR)
30. Kristopher Bryant (23 WAR) 
31. Jose Ramirez (21 WAR)
So here's the problem we're running into here, which maybe you've already noticed. This is a group of incredibly talented young players, including five MVP winners (Harper 2015, Bryant 2016, Stanton 2017, Altuve 2017, Betts 2018) and two players in Machado and Harper who are on a path that's more "historic" than merely "impressive."
They won't all get there, of course. Maybe Rendon remains perpetually underrated into retirement, or someone gets hurt or flames out at age 30. We've seen all that before -- just look at Wright, Hernandez, etc. 
No, the problem is that we just hit 31 names. We haven't included current stars like Aaron Judge, Christian Yelich, Madison Bumgarner, Andrelton Simmons, Alex Bregman, Luis Severino, Cody Bellinger, Carlos Correa, Trea Turner, Corey Seager, etc., or last season's breakouts like Ronald Acuna Jr., Shohei Ohtani, Walker Buehler or Juan Soto. We haven't had a chance to note the future stars we expect to see debut in 2019 like Vladimir Guerrero Jr., Eloy Jimenez or Forrest Whitley.
We didn't get to consider injury-shortened debuts from high-level talents like Michael Kopech or Victor Robles, or wonder if Player Page for Max Muncy's magical breakout was the start of something historic. We haven't talked about the disappointing debut or out-of-nowhere guy you didn't notice in 2018, like hey, what if Scott Kingery really is great? What if Lewis Brinson figures out how to make contact?
There's just too many incredible talents in the game right now to list them all, which is what makes this entire exercise difficult. It's what makes it fun too, we suppose. It's so hard to predict the future. You'll never be able to know for sure which players on the field are future legends.