I have thought a bit more about modifying creatures:
sprite:attacks on 3 cases (I think sprites became the weakest level 1 unit since equilibris)
leprechaun:always lucky,immune against misluck
elf:damages 3-5 (it's a lot,but in another way,they are hard to get,so they should be powerful...)
satyr:always good morale,immune against sorrow
faerie dragon:random harmful spells(like the beholder)
dwarf:once per battle,they can repair a mechanical unit(1 HP per dwarf)
I think it would be a spell.
magi:cast dispel when attacking(or cancellation or steal enchantment)
titan:immune against malefic spells and habilities affecting mind.
dragon golem:20 or 25% chance to stun when attacking.
balista:move on adventure map:17,doesn't affect the morale of friendly units(should be implemented,it's like if in heroes 3,your units from the castle lose morale because you bought a balista in a dungeon)
crusader:growth 5,cost 650
monk:60 HP,spellcaster(exorcism,death ward,necromancy ward)mana:5
angel:increase morale of friendly units by 1(I also thought about hability to cast on theirself a sanctuary which would last one turn,but I think it would be very powerful)
champion:attack:26,defense:26,increase damage against dragons(faerie,black,bone and mega,don't know if it should work against dragon golem,because it isn't a true dragon)
zombi:can cast shout of death for cost 3(decrease the morale of all enemy units by 10 during one turn)mana:5
cerberus:attack 19,defense 19,cost 210
gargoyle:26 HP,50% magic resistance
mummy:can cast wasp swarm,mana 6,growth:7
venom spawn:immune against poison
bone dragon:the power of the aura depends on the morale of the targeted unit.if it has a good morale,it has a chance to resist the aura and to retaliate.if it has a bad morale,it also loses some initiative this turn(or even skips its following action)
devil:negate first strike,can cast armaggeddon(66 damages)keep mana 10,so it would be powerful,but hard to use.
rogue:poisonous attack(0.5 damages per rogue)
pirate:no boarding/unboarding penalty,can move further on sea
troll:can stun(something not too powerful of course)
efreeti:can cast smoke,the damages done by the fire shield only depends on the number of efreets,and not on how many damages the attacker does.
nightmare:decrease morale of enemy units by 1
nomad:increase the scouting ray by 1
cyclop:can stun(in melee)
behemoth:destroy a door in one hit
thunderbird:surrounded by stormy clouds(gives it +50% defense against range attacks)