I am working within the parameters of CW and when the show began.
I'd pick Charlie Hunnam or Travis Fimmel, but they were leads on Sons of Anarchy and Vikings respectfully at the time Arrow started. And CW probably doesn't have the money to lure them into starring in two shows at once.
Chad was available, and Yvonne in this hypothetical doesn't go to Dexter in the fall.
The Emperor has no clothes.

I thought your boy was Charlie Hunnam? Traitor!
Do you mind if I have a piece of your thigh? It would complement this fine '76 Shiraz

Reboot it on Netflix and hire more professional actors. The only people I would keep and Ramsey,Blackthorne and Thompson as the main cast and will never kill any of them. SO I agree with you on them. Anyone else can be played by any good actor other than those we have now. To be honest that guy who is playing Iron fist looks like Oliver with that beard minus the hair lol Besides he is a better actor and can fight tooso..

Like if we were still on CW?
Can we change characters and story?
Off the bat, just the leads, I'd cast Chad Michael Murray as Oliver. I thought he was just another pretty boy CW lead but he did some good work, even PTSD work, on Agent Carter. Yvonne Strahovski as Laurel.
Definitely keep Ramsey, Blackthorne, and Thompson. Though I'd tinker with the characters all around.
The Emperor has no clothes.

Obviously the same people who direct Netflix Marvel shows.don't know their names, but anyone of them will do a much better job. But most importantly the writers are the problem here.