There’s a chance that Deathstroke in Season 5 could be another hallucination or even a dream, since we haven’t heard that Manu Bennett is back on set to reprise the role of Slade; a non-speaking hallucination could enable Arrow to bring back Slade as Deathstroke without actually needing Bennett.
The big question now is which incarnation of Deathstroke will be showing up in Season 5.
He’s spoken out about his disappointment in the treatment of Slade’s character, so Arrow may not want him back on board.
The mystery man in Stephen Amell’s photo is big enough that he could easily be Slade Wilson.