My personal choices would be:
Purple – Guardian Panther or Alfrike. Panther is boss as in just about every setting. Will pair perfectly with your current maxed 5* purples. Alfrike as an option cause she’s just a monster when she fires off.
Yellow – My only hesitation with saying White Rabbit is that you already have a Yellow Defence Down hero in Costume Vivica… I would possibly be inclined to suggest Onatel for some boss-as mana control.
Blue:
Would probably suggest Frida for the elemental defence down. Not quite as amazing as Panther is but still super useful in a blue stack. Will accentuate the awesomeness of Finley, Magni & Alasie.
Ariel is a close second with Alice next IMHO.
Green:
Tarlak. No question as you don’t have a better Tile Buffing hero. Read more about my preferences/ order on the Damage Boosters in this link: Titan Damage Boosters: Miki vs. Tarlak vs. Ranvir vs. Wu Kong vs. Guardian Gazelle
Tarlak was the OG improved damage booster over Wu Kong & will also reduce your frustration due to wu kong missing. 
Red: Agreed on Tyr here. Awesome Red sniper