I think it's better to base it on the third game, but to add variety, include similar-style units from other Heroes games in the corresponding deck.
Example:
Diamonds deck (Castle):
2 – Peasant
3 – Archer (Heroes 2)
4 – Spearman
5 – Squire (Heroes 4)
6 – Crossbowman
7 – Swordsman
8 – Monk
9 – Cavalier
10 – Champion
Jack – Paladin (Heroes 2)
Queen – Countess (use the image of a female lord hero from Heroes 4)
King – General (use the image of a general hero from Heroes 4)
Ace – Archangel

Clubs deck (Stronghold):
2 – Goblin
3 – Wolf (Heroes 2)
4 – Berserker (Heroes 4)
5 – Wolf Rider
6 – Boar Rider
7 – Orc
8 – Troll
9 – Ogre
10 – Roc
Jack – Cyclops
Queen – Amazon (female barbarian hero from Heroes 4)
King – Barbarian (barbarian hero from Heroes 4)
Ace – Ancient Behemoth

Spades deck (Necropolis):
2 – Skeleton
3 – Skeleton Archer (Heroes 5)
4 – Zombie
5 – Ghost
6 – Wraith (Heroes of Might and Magic)
7 – Poisonous Slime (Heroes 4)
8 – Vampire
9 – Mummy
10 – Lich
Jack – Knight of Death
Queen – Death Lady (Knight of Death hero from Heroes 4)
King – Lord of the Dead (Lich hero from Heroes 4)
Ace – Phantom Dragon

Hearts deck (Rampart):
2 – Leprechaun (Heroes 4)
3 – Satyr (Heroes 4)
4 – White Tiger (Heroes 4)
5 – Centaur
6 – Dwarf
7 – Elf
8 – Sniper
9 – Dryad
10 – Pegasus
Jack – Unicorn
Queen – Forest Queen (female ranger hero from Heroes 4)
King – Druid (druid hero from Heroes 4, but not a black person)
Ace – Golden Dragon

Diamonds deck (Fortress):
2 – Gnoll
3 – Lizard
4 – Serpent
5 – Boar (Heroes 2)
6 – Rogue
7 – Nomad
8 – Mantis (Heroes 4)
9 – Wasp Nest (Heroes 4)
10 – Wyvern
Jack – Basilisk
Queen – Witch (female battle mage from Heroes 4)
King – Master of Creatures (battle mage from Heroes 4)