The Mets have made the first major splash of the Brodie Van Wagenen era, acquiring Robinson Cano, Edwin Diaz and piles of money from Seattle in exchange for Jay Bruce, Anthony Swarzak and prospects Jarred Kelenic, Justin Dunn and Gerson Bautista. It's a complicated, entertaining deal. There's a lot to unpack here.
This kind of trade has major implications not only to payroll but also roster flexibility, with questions about how to fit Cano on the right side of the infield with younger players Jeff McNeil, Peter Alonso and Dominic Smith, and how the 36-year-old Cano will age over the next five years. That last part shouldn't be a 2019 concern, at least, with Cano coming off a .303/.374/.471 season, and Diaz is a dominant top-five reliever. Whatever this means for the Mets' future, it makes them better in 2019, potentially largely so.
It also makes one thing clear: The Mets are going to try to win now. The National League East is a wide-open, four-team race, and New York wants to be atop it at the end of the season. That's all great, but there's still one remaining question: Now what? 
In 2018, the Mets were a 77-85 team. It's fair to note that some of that was due to a nightmarish 5-21 record in June, and that they finished strong with an 18-10 record in September, but they were also outscored by 31 runs, meaning their below-.500 record wasn't undeserved. They're currently projected at FanGraphs to finish 82-80 in '19; while that ought to go up somewhat once Cano and Diaz are added, that's a number which is both "improved" and "not good enough."
It's not terribly difficult to see where the weaknesses are, either. In no particular order, the Mets have concerns to address in these areas:
1. Catcher
2. Relief pitching
3. Rotation depth
4. Outfielder 

Fortunately for the Mets, the winter is still just getting started. We're a week away from the Winter Meetings, and four months from Opening Day. The roster they have now is not the roster you'll see on March 28 when they kick off the 2019 season in Washington, D.C., though that's also true for the Braves, Nationals and Phillies. Understanding that the payroll is not endless and that they aren't going to just go sign Bryce Harper, Craig Kimbrel and Manny Machado right now, what should they do next? It can't be "nothing." You don't make a move like this unless there's more to come.
If those are the problems, there must be solutions. Let's help with some ideas. 
A catcher
Let's start with the obvious place, because the Mets don't have a strong catching situation right now, nor, really did they have one in 2018. Mets backstops hit just .202/.289/.353 in 2018, offering little offense and bottom-third pitch framing, which earned them bottom-third value. Though they tendered a contract to Travis d'Arnaud on Friday, he might not be ready for Opening Day as he recovers from Tommy John surgery, and neither Kevin Plawecki nor Tomas Nido are full-time options.
So, who? It's probably not going to be J.T. Realmuto, now that the Mets just traded two of their best prospects. It won't be Yan Gomes, who was traded from the Indians to the Nationals. It's easy to say Yasmani Grandal or Wilson Ramos, the only two starting-caliber free-agent catchers available, but we might need to think smaller here since we have larger things in mind below. 
For this spot, we propose Martin Maldonado. No, he won't add much offense (.225/.276/.351 in 2018), but he's regarded as a strong defensive catcher, and published reports have indicated that "defense is the priority at catcher," which might rule out Ramos. This isn't a big name or a big bat, but small moves matter, too. Maldonado ought to be available on a one-year contract.
Another reliever (at least one) 
Diaz is fantastic. Seth Lugo and Robert Gsellman were each valuable in 2018. It's a nice trio to have in your bullpen. Now think about who the fourth-best Mets reliever is right now. Really think about it, try to come up with a name. Is it ... Tyler Bashlor? Drew Smith? Paul Sewald? Daniel Zamora? It's not that they don't have talent, it's that you need those to be the guys you circulate through as the season goes on, not the names you count on to fill half of your bullpen on Opening Day.
The Mets aren't unaware of this, obviously:

Fortunately, the market is flush with good relievers. Player Page for David Robertson or Andrew Miller, as that report mentions, both make sense. So does Zach Britton, Joakim Soria, Justin Wilson, Joe Kelly, Kelvin Herrera, old friend Jeurys Familia or some combination of any of them. (But probably not Kimbrel, who reportedly seeks a six-year deal.)
Our suggestion: Adam Ottavino, who is not only coming off a dominant year for the Rockies, but is also a Brooklyn native. Then, it might make sense to also add a multi-inning reliever for addtional depth. Perhaps Adam Warren, who has a 2.72 ERA over the last two seasons, would be a good fit.
Either way, the Mets need at least one reliever, and probably more than that -- perhaps also a lesser-priced one via trade.
Add rotation depth, don't subtract it
Here's an interesting list of names: Corey Oswalt. P.J. Conlon. Chris Flexen. Andrew Gagnon.
That group made 16 starts for the Mets in 2018, combining for a 6.45 ERA, and that doesn't even count four poor starts from Matt Harvey and one Jerry Blevins start on a bullpen day. The Mets' rotation has three big names in Jacob deGrom, Noah Syndergaard and breakout starZack Wheeler, a pair of frustrating lefties in Steven Matz and Jason Vargas and ... potentially little else. It's a problem.
As great as deGrom was, it's hard to expect a repeat of his incredible season, and both Syndergaard and Wheeler have had plenty of health issues in their past. The point is that they need more depth, not less -- it's what makes the Syndergaard trade rumors more difficult to resolve -- especially if you look at teams like the Dodgers and Astros, who routinely succeed in large part because of their overwhelming pitching depth.

We'll be honest: It's incredibly difficult to predict who this might be. You know all the free-agent names, like Patrick Corbin, Dallas Keuchel, J.A. Happ, Charlie Morton and Nathan Eovaldi. (We've also advocated for Lance Lynn, who is suddenly an interesting option, and there are secondary options like Marco Estrada.) You could envision a trade for Madison Bumgarner or an attempt to get, say, Marcus Stroman out of Toronto. You've even heard about the possibility of getting Corey Kluber. There's so many ways this could go.
This is less about the names than the point. With Syndergaard, the Mets still need a starter. Without him, they might need two. This is a top-heavy group without enough depth.
Add an outfielder, or someone who can approximate one
Moving Bruce clears up a roster logjam, but it also creates a question: With Yoenis Cespedes likely to be unavailable until after the season begins, the Mets' starting outfield consists of Brandon Nimmo, Michael Conforto and ... who? Smith doesn't seem like a viable corner-outfield option, and Juan Lagares' inability to stay healthy and hit consistently at the same time makes it difficult to count on him every day, despite his strong glove.
While it's possible they just sign the still-valuableAndrew McCutchen or someone like Nick Markakis or Michael Brantley, the return of Cespedes makes that four outfielders for three spots. Instead, why not someone who can play outfield when needed, but can also fill in all around the diamond in today's multi-positional game?
You know who I'm talking about: Marwin Gonzalez.

Gonzalez basically hit like Machado over the last two years; while obviously no one thinks he's the same caliber of hitter, he's an above-average bat who started at five positions for Houston in 2018. He's also a switch-hitter with no obvious platoon split, important because Cano, McNeil, Nimmo and Conforto are all lefties, and he provides some shortstop insurance if Amed Rosario's possible breakout takes some time to get going.
This will take a deal of at least three years, because other teams would have the same interest, but he's a perfect fit for a roster that was particularly inflexible in 2018 and he's a big upgrade from the non-tendered Wilmer Flores.
* * *
As you can tell, there are a million different ways to go for the Mets. You can fit the pieces together any way you like. You can even rationalize a Syndergaard trade in which he's replaced by a lesser starter like Happ or Eovaldi if that's the trade that helps you add talent and fill some of these holes. It's not the exact names that matter right now. It's that the Mets continue to try to keep improving.
It's one thing to make a win-now trade. It's another to stop there with a roster that's better, but not ready yet to win. It's a long offseason. The Mets still have much to do.