David Wright, Ike Davis, Travis d'Arnaud, and Ruben Tejada around the infield. They still need to add a 2B to round out the group as well as field an entire outfield. Daniel Murphy is passable at 2B for now, however I don't see him as a long-term solution on a championship-caliber team. His defense is average at best for a middle infielder and his offensive production does not really make up for it. I like Murphy, but as a super-sub or as trade bait.
The first place we can look for that next core piece is the group of current major league 2B. The current top performers at the position are Robinson Cano, Aaron Hill, and Dustin Pedroia. Cano and Hill are free agents after this season but I don't see the Mets investing top dollar in 2B when they have other positional needs. Ditto for Pedroia who won't be available until after the 2015 season when the Mets will hopefully have the position solved. The only way to acquire him at a reasonable cost would be through a trade and I don't see the Red Sox moving him. The rest of the current crop of major leaguers are not a significant enough upgrade from Murphy to justify making a move.
This means that the next 2B for the Mets will likely play the 2013 season in the minor leagues, either for NYM or for another organization. I don't think anyone within the organization currently profiles as the solution either. Reese Havens has the talent but it seems as though his trouble with injuries has derailed his career and he might never make it up to the majors. I've always liked his potential and even liked him more than Ike Davis when both were drafted in 2008. I'll root for him to break through, I just don't see it happening. Wilmer Flores likely has the bat for the position, however reports don't indicate that he'll be able to handle defensive duties; his future in the major leagues is at 3B and likely with another organization (see: Wright, David). Gavin Cecchini might be the answer, but he won't be up at CitiField before 2016 and a lot can happen between now and then. After these three, no one else currently in the Mets' system will be a starting 2B at the major league level.
Therefore, the Mets will have to look at other teams' middle infield prospects in order to fill the position. The top 2B prospects in baseball are Kolten Wong of St. Louis, Delino DeShields of Houston, and Eddie Rosario of Minnesota. All three players should become solid major league regulars, but I don't think any of their teams will look to move them. Wong is the heir apparent in St. Louis, Houston views DeShields as an important part of their rebuilding process, and the small market Twins almost never move their prospects. The best chance for the Mets is to take a look at the SS prospects around MLB with an eye on converting one to 2B.
Of the top SS prospects in baseball, Nick Franklin is the most likely to be available. A potential high-OBP and 20/20 offensive player, there are already questions as to whether he can stay at SS long-term. If he is more of a 2B, his future in Seattle is murky, even if he is one of their two best positional prospects. Assuming Franklin can't stick at SS, Brad Miller is looking more and more like he might be the future SS for the Mariners and erstwhile super prospect Dustin Ackley is still the likely 2B going forward. Since Franklin's greatest value is as a middle infielder, he might be most useful to Seattle as a trade piece to upgrade another position.
He is obviously not untouchable as Seattle was willing to include him in the rejected trade for Justin Upton. The question becomes what would the Mariners want in return for their former 1st round pick. Even though teams seem to always be looking for pitching, the Mariners are as well positioned there as any team in baseball, boasting Felix Hernandez and 3 of the top pitching prospects in the game. They also have one of the game's best catching prospects in Mike Zunino, a powerful young DH in Jesus Montero, and, as mentioned above, their middle infield is likely sewn up with Miller and Ackley. The Mariners need help at the corner IF and OF positions, as well as CF.
The Mets could build a potential deal around Wilmer Flores. The Mariners need help at 3B and Flores could solve that for them. He might need a month or two of experience at Las Vegas, but a solid start there could greatly increase his trade value. Flores and a lesser prospect like Matt den Dekker might be enough to pry Franklin away.
Franklin should be ready for the majors some time around the ASB this summer. His addition at 2B would complete the Next Mets infield and he could even slot in at the top of the lineup. Five of the eight everyday spots would be filled (and cheaply), leaving the team with only a few holes to fill and a lot of flexibility to fill them with.