Season 1 started badly, and got significantly better. Season 2 started great, and ended horribly. I am half way through season 3, and it has been a complete disaster. My opinion, but I don't trust them with much. Especially after they killed of their best character and are now bringing her back because they messed up so badly.
The Joker and Harley are clearly going to be in the Batman solo. They are setting it up at least for the Joker already. And my money says where the Joker goes, Harley follows. And you better believer there will be a Suicide Squad sequel if it makes bank.
I think Arrow is not your kind of show. Because i find the writing excellent in Season 1, and Season 2. Season 3 had bad writing. When the easter egg was shown of harley, all the producers were excited and thrilled by it. So they would treat the character with utmost respect it deserves.
Rumors are they will make a red hood movie adaption. I don't know if thats legit, and joker appears in it, but not harley. So i can't see Harley following Joker around everywhere.
I hope the Suicide Squad movie fails badly, and it's a one time movie only. That's what they should get for screwing Arrow, by taking Deadshot away, when it was most needed in the HIVE storyline in Season 4.