What you're after is Versatile Duelist - Dragon 381.
For one, it grants proficiency with all one-handed military heavy blades. Secondly, it lets you use "a one-handed heavy blade" instead of a light blade with any rogue power that requires a light blade. You can also deal sneak attack damage with the power.
Yes, this is better in many ways than the Eladrin racial feat Longsword Finesse (from Martial Power). How? Well, let's see...
1. You don't have to be Eladrin.
2. You can use ANY one-handed heavy blade with rogue powers, rather than just longswords.
3. It doesn't gimp your Sneak Attack damage by one die.
On the other hand, since what you want is basically a sabre, I'd suggest simply reflavoring a rapier. There's no reason it can't be slashy. You could, of course, make a new weapon, but since 4e no longer distinguishes between slashing and piercing damage, it would be statistically identical to a rapier. So just use the rapier stats and call it a sabre.
If you want different stats, the Kukri is a +2 prof, 1d6 dmg shortsword with the brutal property. The Katar is a +3 prof, 1d6, high-crit weapon. Both are superior weapons (and, yes, light blades).
Between those two, the rapier and the shortsword, I'd say the bases are all pretty well covered, statistically speaking.