Who should i complete my team with ?

Try JAlter + Waver + Hans. Hans is one of the best support for JAlter, or if you want a tank you can even try with Leonidas.

If you are using the team for events use Jalter-Waver-Class advantage Aoe servant.
For full support Jalter you could do.double waver+Jalter and or Nightingale or Shakespeare for Buster buff.

Depending on what you're fighting, the team comp will change. Luckily, your list of servants are pretty versatile so you can build quite a few.

For farming situations, I would go Artoria Alter (both) and Waver, assuming they have some type of NP help such as kaleidoscope. Otherwise, pair one of them with Jalter for the buff dragon attack. Saber Alter is better in this case, as Lartoria will take a lot of stars that will be mainly used for Jalter.

In ST/Boss scenarios, Jalter + 2 Wavers are nice, even more so if 1 of them has Fragment for constant star support. You can go for Hans if you want with the Chaldea Combat Uniform. Start with Hans, once you get enough stars, buff with 1st skill, switch in with Waver, then continue buffing and then wreak havoc. Getting another reliable star generator like Jack makes it easier to setup, at the cost of lesser buffs.