I wanted to make a Duelist class using the latest editions of Martial Power.
A typical fencer who wields a single rapier (perhaps a three-pronged dueling dagger too)
I guess it would implement the Acrobatic Rogue and the Tempest Fighter builds from Martial Power, and perhaps the Brawler from Martial Power 2 (for the bonuses of having one hand free).
The main idea is an character who can deal damage fast and has good survivability due to agility.
There was a dragon (#381) magazine article that tried to turn the 4e Rogue into a duelist. Of course, you have to have DDI to look at it.
It's an archetype that D&D has never really done well or tried to tackle as a core class, it's always an NPC class or PrC or build or kit or whatever, and it rarely worked well because of the system's dependence on armor for defense.
4e really lessened armor dependence, so a duelist would have been possible, if they hadn't declared that weird moratorium on new martial classes, thus limiting MP to new builds.
The light-armor/light-blade-using Rogue is the best candidate for a duelist. It's a striker, so damage is potentially good, and it's light-armored and agile. Some of it's exploits support the duelist flavor, but a lot of them really don't, so you'd need to be very selective. The main problem is generating CA in a 1:1 duel.
Fighter, being STR-based and a defender isn't such a good candidate. The ranger is a light-armored striker and a TWF fiend, but the TWF build is STR-based and has a hard time maintaining that light-armored agility. Warlord, of course, is right out.
There are also two sub-classes that are candidates. The Slayer makes an unlikely duelist, lacking any sort of exploits or finesse, but a Melee training feat makes it DEX-only SAD, and it does have a 'duelist stance' that'd be quite good for actual 1:1 duels. Even throwing away it's two-handed-weapon and heavy armor features it'd be a pretty deadly rapier-wielder. The Theif is the other option, it's good with a Rapier once you get proficiency, is light-armored, agile and can get CA without much help. But, like the Slayer, it lacks cool exploits to support the archetype.
If Rogues could take Thief 'Tricks' or Thieves take Rogue exploits you could cobble together a pretty good duelist.
Something like a Duelist might, oddly, have made a decent take on a melee-oriented martial controller (perhaps as one of several distinct builds of a broader class).
Actually creating a Duelist class would be a daunting undertaking under the 4e AEDU model. While the Essentials basic-attack-spamming martial approach would be a lot easier, it'd be hard to do the archetype justice with it.