I think it would be good idea to add dual type mechanic similar way as in pokemon games.
For example Alastair could become “Undead Knight” and be treated both as “Knight” and as “Undead” for purpose of all skills & abilities.
He would be then affected both by “deal double damage to undead” and “deal double damage to knights” enemy skills.
He would be also counted both as a “Knight” and as “Undead” for purpose of team bonuses.
Other examples of current creatures that might be affected by this idea:
Bone dragon: Undead Dragon
Paladin: Divine Knight
Brian the Lucky: Human Knight
I think this change would make creature type more important and it would open way to new strategies & creature combinations.