Next servants

The most obvious choice is Merlin, because of how stupidly good he is. However, from my understanding, he mainly benefits Buster teams. Given your current setup, if you want something to complement it, I'd go with Altria (Archer). Having a versatile class lineup on your arts team will help make sure it stays relevant against more enemy lineups - Waver is also super easy to find as a support.

If you ever want to build a buster team, though, Merlin is hands down the best pick there, and you'll have plenty of time to save for him.

While Merlin is amazing, he isn't coming until the very end of this year, and even then he isn't limited time. If you want to ensure getting him right away as much as possible, I guess you can start saving now, but he isn't really imminent at all.
Waver similarly is amazing, but he also isn't limited time. He is going to have a rate up pretty soon during the Fate/Zero event with Iskander.
It seems like the most limited servants you want are Artoria Archer and Tam Lancer, both of which will be coming this summer at the same time, but on two separate summoning focuses. If you'll need to choose between anything, you'll need to choose between them because there will literally be no time to save between them and you will have to prioritize at least one.
Scathach will be coming a few months before the swim event, which will be in the late summer. She comes back pretty regularly, but it will be a while after this.

Truth to be told, it all depends on your priorities. If you mostly want servants for their usefulness focus hard on Waver for now, be ready for his rate up summon probably coming late this month, and then start saving for other things. He's the best servant in the game right now, and him + Tam in an arts team is basically unlimited support works. He's a monster with an arts setup. At the same time, to a lesser degree you could see Tam as already filling that major support role, but either way, Waver is amazing.
If you really like rare servants who are of characters you like, your top priorities should be the beach servants and then maybe also Scathach (although Scathach is also incredibly powerful herself but not as much in an arts team). If this is the case, I say forget Waver for now. Throw everything on Scathach in a few months and then immediately start saving to try for at least one of your beach servants.
Don't worry about Merlin yet. I'm sure even if you throw everything at the beach servants you'll have decent time to save for Merlin, and what's more, even if you miss his initial appearance he'll be hanging around.

And remember: save your tickets!

For Lancer, I'm more into Scathach. Tamamo Lancer is not bad, but 2 of her skills have demerit and might cause you troubles.
Artoria Archer is amazing NP spammer. But, if you're only looking for arts archer, there will be free welfare Kuro on prillya event. Kuro is also one of the best archer
Waver vs Merlin. I'd say go for Merlin because he's limited and Waver can spooks you anytime