Also the "backstory" between Talia and Oliver they told in a matter of seconds... and then yeah, nobody who is just watching the show understands why on earth Nyssa now has a sister and why they are at odds with each other. They didn't even try to explain, or did they … except perhaps alluding to "father always liked you better" or something.
It was so lacklustre.
And yes, Samantha was a complete throwaway and I don't really understand what for – it's not like we'll see much of Oliver being a parent, I don't think. That'll be a plot point for a few episodes until he decides "his life doesn't go well with child rearing and it's too dangerous" and then he'll send William away to live with relatives. I suppose the actress who played Samantha will be the next big bad on season 3 of "Shadowhunters" but then again, it's not like her role on "Arrow" was pivotal in the sense that she had to appear overly often.
I'm not sure if Willa isn't available as such or if she is just over it. It's not like they really give her good storylines. Otherwise it doesn't look like she has anything else lined up.