The Suicide Squad helped Team Arrow way back in Season 2; maybe we’re in for something on an even larger scale in Season 5.
Wendy Mericle’s comments indicate that we’ll be pretty surprised, so I’m guessing that we’ll see a team-up of some pretty unlikely characters.
A big question is who Oliver will work with to defeat Prometheus.
We already know that Katie Cassidy will return as Black Siren by the end of Season 5, and Oliver has quite a gallery of rogues who might be convinced to join him in taking out Prometheus if properly motivated.