Thanks to Ivan for the heads up.

Update: More information on Colton's return has been revealed.
“We’re thrilled to announce that Colton Haynes will be reprising his role as Roy Harper for a short arc on ‘Arrow’ this year,” executive producers Marc Guggenheim and Wendy Mericle said in a statement, adding that the reason Roy returns when he does is to “help resolve an urgent matter with high stakes for the team.”
Although his visit will be “short,” it will turning into “something surprising when he reunites with Thea [Willa Holland] and sparks a change in both of their lives that will have long-lasting consequences.”
Thanks to Dumble for the heads up.
Roy's coming back very soon 🤗
— Colton Haynes (@ColtonLHaynes) September 9, 2017