I understand your question as, when you do an elemental summons, what color?
You are in good shape on purple, blue and red. Rigard is a great healer, removing status effects. He pairs nicely with Sonya, who strips buffs from foes and does nice single-target damage. Boldtusk is widely viewed as a top-shelf 4*, he gives you the option of using Thoth instead of Rigard.
So that leave yellow and green:
Yellow you have Wu, a seriously great hero but, by design, unreliable because his special either secures the win for you or blows your window of opportunity. Chao is my least-favorite 4* yellow. A yellow pull could improve your daily bench here with Li Xiu or Hu Tao (not everyone’s favorite) or a 5*.
Green while some love his high attack stat and the attack debuff (esp against titans), I see Caedmon and Melendor much more often in serious teams. Caedmon is very similar to your Sonya, tho, and Melendor is another healer (and you already have Rigard and Boldtusk). If Thoth is going to be your lead purple, then Melendor would be a nice add.
I’m intentionally omitting any consideration of your drawing a 5*, as the odds are so low and any will add to your bench.