The first middle infielder I would have interest in would be Aaron Miles, formerly of the LA Dodgers. Last year he hit .275 with 3 HR and 45 RBI and made only $500 K last year. He can play shortstop, which has on a regular basis. If he got a raise, the Mets could still bring him in for less than the mandated $1.5 million.
Julio Lugo once was considered a top shortstop. Coming out of the Astros organization, he was one of the better all around SS in the National League. Now at age 36, he is not the same player but may have something left. He may be worth bringing in the spring training on a minor league contract. After having a decent year as a back up infielder for Baltimore in 2010 (.249, 0, 20 in 241 ABs) he didn't help the Braves much towards the end of 2011. (6-44, 3 RBI) It couldn't hurt to have him show what he has left, as the Mets can cut him if they don't like what they see.
Christian Guzman has been out of baseball since 2010. He may want to retire at age 34, but similar to Lugo, was one of the top SSs in the game in his prime. He seemed to be ok with the part time role he had over the last couple of years with the Nationals and Rangers. He is a natural SS who can play 2B if needed, still has a little speed and can work the count very well. Since 2007, he has consistantly been a .280 hitter until he went 7-46 in September with the Rangers in 2010. Also similar to Lugo, they don't have to commit much to him and can let him go if he fails to live up to expectations.
Adam Everett will be 35 before opening day. He may not have anything left being he played in the majors sparingly for the Indians last year. And its been a nightmare for him since the Astros let him go after the 2007 season. Never much of an average hitter, his two best years were 2005 (.248, 11, 54 in 152 games) and 2006. (.239, 6, 59 in 150 games) After making 14 errors in 2005, he made 7 errors the whole 2006 season. Even last year, he made the plays in his limited time. The problem is he may not be able to hit any more. That will not help him, but he would take a minor league contract.
Everybody remembers Eric Bruntlett as the 2B that caught Jeff Francoeur's unassisted triple play. He hasn't played since 2009, but is somebody that may be looked into. Alfredo Amezaga was a very useful player for the Marlins from 2006-2008. He did very well at the super utility player. Both could be had for nothing.
The FA infielder that intrigues me the most is Willie Aybar, (who will be 29 by opening day) who most recently played for the Tampa Bay Rays in the 2010 season. I don't know why he didn't play in the majors last year. He was an asset to the Rays from 2008-2010, hitting .253, 10, 33 in 324 ABs in 2008, .253, 12, 41 in 296 ABs in 2009 and .230, 6, 43 in 270 ABs in 2010. These are numbers you can take for a backup middle infielder. He is a talent but he has had some off the field issues that have derailed his career.
I personally think the Mets should bring in Aybar or Miles over Wilson and Cedeno. I would consider Theriot, but understand that his price may be too high for the Mets. The others I mentioned all have proven that the can be useful at some point in their careers. To me, Everett, Lugo and Guzman all have had much earlier success similar to Wilson. The thing is, they all have struggled in the past several seasons, which puts them in equal ground. Cedeno should not be considered at all, and come to think of it, neither should Bruntlett or Amezaga-though both pestered the Mets.