Sweet Roster! My 2 cents times 5 below.
Red: Costume Khagan. He’s such a hard hitter and adds defense to the team. Well rounded hero, only drawback is Slow mana.
Blue: Glenda. She may not be the best hero on paper, but in a blue stack she is going to up the power moves of that strong blue line up you have. Honorable mention, Raffael. He’s amazing for rush wars and he has saved by bacon a LOT in the ninja tower.
Green; Fogg and it’s not even close. Mother north is amazing but you have Alby. Plus you don’t have Evelyn, you desperately need a green elemental defense down. Fogg fits perfectly.
Yellow: If you have 12 darts and plenty of D Blades and Tomes, do Akkorog. If not, wait.
Purple: It’s gotta be Bera. Flavor of the month for defensive tanks and Freya pairs great. Close second would be Lepiota, but Bera is golden.