I like the idea of playing as Dr. Frankenstein or a generic "mad scientist" and all his lab created minions & captured creatures. Feels very thematic.
Unit ideas:
Abomination - On death poisons 2-3 squares which deal damage when walked into
Vampire - heals on attack
Flying gremlin - Flys to other lanes to assist if broken through
Frankensteins Monster - Has cleave (Attacks hit 2 squares)
Mole - Digs underground and attacks the rear and/or directly to the castle
Zombie - After killed rests for X turns then rises up again.
Werewolf - "Infects" enemies. Infected units gain 2/2 but randomly attacks allies/enemies (maybe 50/50 chance each attack)
Clone of Mad scientist - Ranged attacker. Throws beaker with acid / Ray gun etc
Looking forward to seeing what you come up with!