I associate Beast Master mostly with wolves. OK, green animals as a whole but mostly wolves.
Timber Wolves in the original, both in the deck and as starting advantage and potential reward in the next duel, and
Wyluli Wolf was added in the overhaul. Also, look at his animated avatar in-game.
Master of the Hunt and
Master of the Wild Hunt would fit, but they aren't in the game this far.
Another idea is to splash a little white for
Tundra Wolves, Isamaru,
Watchwolf and
Tolsimir Wolfblood.
I am fine with beasts as well - there weren't many (any?) green ones in the original game.
Witch - Agreed. Skeletons, zombies, rats, maybe also bats and ravens?
Undead Knight - Knights, zombie/skeleton, 'unholy war'. If we had the black Paladin cards, they'd belong here too.
Necromancer - zombies, spirits, wraiths
Summoner - yeah, maybe spirits, but also green/nature Forces and Elementals.
Azaar - vampires, demons, liches
Astral Visionary - something large and aquatic. And mind tricks.
Sainted One - Angels, Avatars, Knights and 'holy' artifacts