While Arrow Season 4 was uber disappointing, the good news is that the show sounds like it is going back to its roots with Season 5.
DC All Access caught up with Stephen Amell who offered:
The only thing that I can tease is that it's only a villain we could do if we were in Season 5 of a show because it calls back to what has happened in yesteryear or yester-season as the case may be...
Of course everything is going to change. If you seen the finale, you know there is going to be a whole element for me professionally that we really haven't seen yet, and we have been in this cycle of introducing shows for a couple of years now. Obviously Supergirl is coming to the network, but we don't have to introduce Supergirl. I think we are going to focus a lot on just getting back to some of the stuff that Arrow does really well.
TV Line also has additional info on the villain of Arrow Season 5:
• It's said the new villain will be loosely inspired by Idris Elba’s Wire drug kingpin Stringer Bell.
• The character is tentatively named "Anton Church," described as: "a ruthless crime lord who sets out to fill the sizable void left by Damien Darhk and H.I.V.E."
• The casting notice describes him as an “apex predator” who “cuts his way through the shadows” by taking down “the biggest threat first.”
• Stephen Amell also said the villain won't have super powers.