Arrow is now casting for an "introverted, moody, and naturally athletic" teenager for Season 6, according to TV Line, and they directly connect this casting to Slade Wilson, aka Deathstroke. Arrow star Stephen Amell previously confirmed that Manu Bennet would be back as Deathstroke for multiple episodes this season.
That means that Oliver is going to fulfill his promise to Slade, his friend/mentor/worst enemy (depending on the minute), and help him find his son. The trouble is, we don't know which son that will be.
In the pages of DC Comics, Slade Wilson has three children. His daughter Rose would also fit the description above, which would be a fun twist -- she also followed in her father's footsteps as an international assassin for a time -- but "son" was specified in the on-screen conversation between Slade and Oliver.
That still only narrows it down to two: Grant and Joseph. Grant Wilson is the more likely to appear on Arrow. Slade's eldest child, Grant was the first character in DC Comics to go by Ravager (a title his half-sister Rose would later inherit). He was an assassin who died while going after the Teen Titans, leading to Slade's vendetta against them that has lasted to this day (just this year, they fought it out over the loss of Grant again). He's a no-nonsense, serum-enhanced assassin, much like his father, making him a perfect fit for Arrow after it went "back to basics" a bit last year, toning down the mystical and over-the-top metahuman use that fits better in shows like Supergirl, Legends of Tomorrow, and The Flash.
Slade's other son, Joseph, goes by Jericho and was created by Marv Wolfman and George Perez during their seminal run on New Teen Titans. Jericho has an interesting power set: He can possess people just by making eye contact with them, and once possessed has all their abilities and powers, as well as access to their memories. Usually mute, Jericho can communicate through those he possesses as well. It'd be fun to see Jericho come to life, but again, probably a little out of place on the now re-grounded Arrow. There's also the possibility that Andrew Kreisberg, Greg Berlanti, Geoff Johns, et. al. will want to save Jericho for their new Titans series coming to DC's digital service next year.
Regardless of which Wilson child appears, it's only going to spell trouble, likely for both Team Arrow and Deathstroke himself -- Daddy dearest doesn't have the best relationship with his kids.