Who should I save for in 2019?

there will be First Hassan in 2019, you should pull him to complemented your current line up. He's a rare buster assassin with good skill set too.

King Hassan, Kiara, Musashi, Umu Caster, and Eresh are all good. Check them out and see if you like any of them.

Also, old servants like Waver, Tama, and Gil are always solid picks. Waver will share a rate-up with proto Arthur but Tama and Gil will have solo rate-ups before CCC event.

- Gil has a rate-up together with 2030, so that'll be a good banner to go for. If not Gil, then summer rerun for Altria archer would be good too, since you lack an ST archer.

- Eli isn't that great of a saber, and in 2019 you'll have three great ones to choose from: Musashi, Okita and Nero Bride. All three are good for different reasons, any one of them will be worth rolling for.

- As for lancers, summer 2019 has arguably the best ST lancer on rate-up: summer Raikou. The best part is that she is SR, so it'll be presumably easier to get her than an SSR servant.

- You need a good assassin. For ST, King Hassan is probably the one to go for. Jack is better imo, but not by all that much, and if I remember correctly she doesn't have a rate-up next year. As for AOE if you are interested, Cleopatra is generally regarded as the best.

- I noticed you didn't mention berserkers. If you don't have him, Cu Alter will be on rate-up in the Chaldea boys banner. He is really close to Amakusa Shirou and Waver, both really good, but since you already have Merlin I'd say Cu Alter is more worth it for you.

The above listed are some ideas to fill some holes I can see in your current lineup. All in all, for specifics, I'd say the best banners to roll on are: King Hassan rate-up, Gudaguda (Okita) or Shimosa (Musashi), Chaldea boys, Gil + 2030 banner, and summer 2019, specifically the banner for lancer Raikou.

Saber : Munenory Yagyu, i would say he's the same level as Saberlot and very strong.
He has high damage and pretty tough, really good when clearing harder content and challenges.
Musashi for the Buster.
Assassin : Jack or King Hassan, Shuten for AOE debuffer.
Berserker : Penthesilea, a crazy Burst damage zerker and Hijikata for ultra high crit + NP damage.
Hijikata is like Raikou but ST SSR Buster Crit zerker, Chu Alter if you lacking an anchor.
Lancer : Raikou, great lancer simple yet strong.

For welfare zerker, there are Guda-guda 2019 if i'm not wrong Chacha is in there.
Chacha is a good pick if you lacking AOE zerker.