Well seeing as it's a finesse weapon as is, as long as you have proficiency with it, you can use it for sneak attack and don't really need a rebuild... however, if you want to rebuild anyway and get some use from it, swapping over to high elf (for the elven weapon's training (longsword in particular), booming blade or greenflame blade cantrip (your choice)), that would work as well. Build wise, it would probably look something like:
Race: High elf
Class: Rogue (swashbuckler) 4
HP: 35
Str: 8
Dex: 18 (15+2 elf, +1 ASI (level 4))
Con: 16 (15 start, +1 ASI (level 4))
Int: 10 (9+1 high elf)
Wis: 12
Cha: 12
Background: Charlitan (selling himself off as the best lover/ swordfighter/ defender of a lady's honor in the world... or whatever.
) for Deception and Persuasion.
Skills: Acrobatics, Deception, Intimidation, Perception* (elf), Persuasion, Slight of Hand, Stealth*
((* means expertise))
Tool prof: Disguise kit, Forgery kit, Thieve's tools
At later levels: Level 6 put expertise in Deception and Persuasion (to max out Panache potential)
Level 8 get 20 dex
Level 10 get +2 Con or Cha (con because you are a melee rogue, cha because more sexy, dashing swords elf)
Level 12 get either +2 cha or Duel Wielder (for even MOAR sexy duelist elf or for +AC and for the use of an offhand weapon to land sneak attacks better)
Level 16 either max out con (if you added to it before) or +2 cha (if you ignored it this long, might as well just max out your cha and ensure NO ONE resists trying to take a bite out of your candy ass while you taunt them to oblivion)
Level 19 max out cha, or add your last +2 to it. If you maxed out con and got dual wielder for AC instead of adding to cha by this point, you might as well just get it as high as you can to keep your odds as high as you can (a +14 is still ALMOST as good as a +17, after all, especially since most critters still don't have sense motive training).
If you feel like it, swapping out any one of those choices with Linquist for +3 languages is also a valid choice, especially since you then have a wider pool of targets to taunt to death. Not necessary, however.
If you wanted to multiclass a bit of fighter or something in there, well, I'll let someone better at multiclassing guides cover for that.
(a 6 level fighter dip after level 12 would be... decent. Especially as a champion or Battlemaster, one of whom can crit HARD when you get that sneak attack-crit combo (more often than any other rogue), and the other of which who can give you a bit more stickiness, more d8's to roll with your longsword and generates advantage for itself to, again, increase crit chance and increase the odds of landing a sneak attack. Additionally, Fighter is even better if you decide to NOT go the dual-wielder path and instead wield your finesse longsword two-handed. You can have a d10 dex weapon, grab the great weapon fighting style, and reroll all that sweet, sweet sneak attack damage for a 1 level dip (though 2 levels at least is recommended if you grab any fighter at all, and pushing it to 6 for 2 ASI's, Extra attack and a subclass is also worth it for the most part).
