There is a strong idea in every season of Arrow. At least one main character stops being series regular for some reasons and leaves the show or just appears as a guest next seasons. Here are previous examples:
Season 1 - Tommy Merlyn (deceased)
Season 2 - Moira Queen (deceased) and Slade Wilson (caught)
Season 3 - Roy Harper, ("deceased", left the team anyway)
Season 4 - Laurel Lance (deceased) and Malcolm Merlyn (became bad guy again)
Season 5 - ?
Now who do you think either will die or leave the team from the main cast this season?
Oliver Queen - Oh, please.
Felicity Smoak - Nope, this team needs a hacker.
Thea Queen - Nah, they already tried to kill her and resurrected, won't work again.
John Diggle - Unlikely, he is badass this show needs.
Curtis Holt - Probably, he does not seem no have huge role in the series.
Adrian Chase - It's strange they presented him as a main character because he does not have so much screen time, so i guess we will not consider him as a full main character. But his Vigilante alter ego won't make this season, i'm totally sure about that.
But my opinion is that none of then have the highest chances to leave this show. There is another non-mentioned persone from the main cast.
Yep, this guy became totally useless. Lance was good previous seasons but come on, not now. All he does this season is complaining about death of Laurel and drinking. They are probably planning to kill him and that's why they just making this character pointless for the whole show so we won't miss him that much. I guess, soon we'll have to say goodbye to buddy Quentin.