1)
http://img4.wikia.nocookie.net/__cb2.../Siegfried.gif (Z/Siegfried)
Z/Siegfried is a semi-famous thief and swordsman in FFVI who steals many items, usually a couple of minutes before the party arrives to the dungeon. That or he comically challenges you, then runs away, mid-battle, stealing all of the treasure mid-flight. In this sense, he's similar to Gilgamesh, who was already absolved of the crimes.
There has also been a long held debate on who Z/Siegfried is in FFVI, as the real Siegfried in the coliseum states that Ziegfried likes to impersonate others. This might be a stretch, but the card issuing the challenge to Hildibrand in the earlier cut scenes looks to have a slightly mangled form of the Eorzean letter "Z" on the card as well.
The downsides to this theory is that Ziegfried/Siegfried has never been known to be a womanizer -- the current story line looks to be going in that direction, as the thief seems to be a stealer of hearts as well as a general thief. Also the fact that the thief claims to have slept for 400 years, which would be unnatural for any human unless they were in the Void/Interdimensional Rift.
2)
http://img2.wikia.nocookie.net/__cb2...I%29_small.png (Gogo)
There's not much to go on for it being Gogo, outside of the fact that the thief is known to use multiple identities and can impersonate anyone. Impersonation and copying identities should be something a mimic can perform. S/he may have also came out of the same Interdimensional Rift that Gilgamesh came from as well (he cast himself/herself into it in FFV, appeared in Zone Eater in FFVI). The Interdimensional Rift itself alters ones appearance depending on where they exit and is also stops time from flowing inside it, meaning that Gogo could very well be a 400 year old demon as well (he's a demon in FFV).
What more than a famous mimic to be the con artist in question?
3)
http://img1.wikia.nocookie.net/__cb2...Ultros_ATB.png (Ultros)
Ultros is also a stretch, but a likely one due to where the story is headed. The thief has threatened to steal the lapis maiden's virtue, which might not exactly be an item of any kind, but the woman's heart in question (a lapis lazuli is blue, the woman in the 2.3 preview cutscene has blue hair). How Ultros fits into this is the fact that he's a huge womanizer in any incarnation of Final Fantasy where he exists.
The downside to this is the fact that he's not known to be a thief, though he likes to play pranks on people. Which leads me to believe...:
4) Siegfried & Ultros
Sure, why not a combo of Siegfried and Ultros being the thief? Ultros and Siegfried know each other in FFVI as rivals, though the story is not expounded upon, outside of Ultros attempting to steal The Waring Triad statues in attempt to impress Siegfried, a famous thief in his own right. This 2.3 story line could very well end up with the culprit being Ultros, who's impersonating Siegfried as well as attempting to kidnap some fair maiden all the while.
Ultros being a 400 year old being could make sense, though it would be a gigantic stretch to explain how he could change faces and identities, unless it was Seigfried who stole the Treaty-Blade followed up by Ultros coming at the last minute and issuing the challenge. There's also the fact that Ultros is a gigantic purple octopus and not a swordsman or duelist, but again he could be impersonating someone else, namely Seigfried.
5)
http://img3.wikia.nocookie.net/__cb2...ductor-ffv.gif (Enkidu)
In a weird twist, the thief could be Enkidu, Gilgamesh's loyal friend and companion. In 2.2, Gilgamesh spent sometime off screen chasing a "demon bird" which lead to him stepping on Maria's pot and joining the party to find the Treaty-Blade. Enkidu is a demon, so it would fit with the duelist claiming to have slept 400 years, though it would be a stretch to place him as a multifaced opponent.