Arts servants coming up?

Summer Artoria and Mordred.

some of them are not upcoming servants.
vlad 3 is good art berserker especially after interlude quest
sherlock holmes also one of the best art servants
sitonai
shiki assasin but i dont know she will return or not
shuten douji
katsushika hokusai

all the already mentioned +i would add emiya(alter) he is really a great art servant and munenori(great saber but you already have both nero bride and saberlot).
Caster of the Nightless City is also a good pick.

Not sure if Houzouin Inshun counts as an Arts servant (3 quick cards, but an Art NP) but he's a pretty nifty 3* lancer coming up in October.
Also I know you already have 2 arts sabers, but Munenori Yagyu is a fantastic 4* arts saber that's coming up around October as well.

Sorry if these don't help, everybody else already took all the good ones XD

I think I've heard that quick is the new meta in JP?

While not on a Banner per se, don't forget the free 4* we're suppose to be getting later this year! If you want Arts you have your pick from Li Shuwen (Lancer), Gilgamesh (Caster), Nero (Saber), Lancelot (Saber), and Emiya (Assassin) as possible Arts related choices. Of those, all of them are pretty good Servants, except maybe Emiya who while works well on an Arts team, can feel underwhelming at times.

On another note, if you're going after Artoria (Archer) you can also end up with Marie (Caster), since they're on the same Banner. And while she isn't the strongest Servant out there, with a triple Arts deck, and an AoE Arts NP, she can be useful at times as an Arts Caster farmer.

I gonna make the list based by gameplay perspective, here we go :

Saber : you've 3 behemoth of Art Sabers, it is more than enough but if you want more saber, Munenory Yagyu is as broken as Saberlot.
For me, skip adding more sabers. Unless it's for collection, then add for another one.

Archer : Chloe alone is more than enough, if you want more then Emiya Alter, Helena Archer and Squirtoria is a good Archer substitute. Their job is almost the same, for me it would be the best to skip Archer. Is a broken unit like Chloe is not enough ? She alone can rekt Gawain real hard and destroy lots of Zerker and Saber or even neutral enemy bosses like it's nothing. Unless it's for collection, go for another Art ST Archers.
Emiya AOE is good to cover your archer department and i recommended this one.

Lancer : Fionn ( he's not really good but with your party even a bad Art servant like him can be a monster ). For me, just skip him.
Li Shuwen, ST Art crit Burst OTK Lancer. The best Art Lancer now on NA, i recommended this one.
Ereshkigal/your favorite servant take the highest priority, focus only to get her first and good luck !

Rider : Mordred, one of the best AOE Art NP spammer servants with decent survivability. I recommended this one.

Assassin : Shuten is a good debuffer and a good semi-supp for your Art teams, i recommended this one.
Emiya assasin is also a solid ST Art assassin, you can pick between Shuten or Emiya but it's still up to you.

Caster : STOP ! Marie Antoinette and Caster of Sheba for another collection but it's is not really recommended to roll another caster. And also, you got tons of ultra powerful Caster already.

Zerker : Vlad III, i recommended this one because he's simply the best Art Zerker on NA.

Because you requested the servants from each class that you're lacking, i made some list based on your needs.