Hearthstone

Maexxna - At the end of your turn, web (same as frozen) a random enemy minion. Adding the ability to break webs would differentiate them from frozen effects but might be too complicated for hearthstone.

Loatheb - All healing effects from both players are cancelled while in play.

Four Horsemen - Silver Hand Knight meets Al'Akir. 10 mana, summon 4 3/3's. Sir Zeliek (former Paladin) with a divine shield. Lady Blameaux (caster) randomly shadowbolts like a demolisher or rag for 1 (2?) damage. Thane Korth'azz (dwarf) with taunt. Baron Rivendare (sup Deathcharger) with charge. 10 mana keeps it in check but might still be OP. Could even make them more synonymous with existing cards. Zeliek = Scarlet Crusader. Blameaux = Demolisher. Thane = Ironfur Grizzly. Rivendare = Stormwind Knight. That sounds even more broken though. 13 mana worth of cards cast for 10 and only takes 1 card slot.. Might have to make them separate cards to actually balance them. Could also be 1 large minion with stacking dot damage to your opponent each turn they're in play (1 damage at the end of turn they come out, 2 damage if alive at end of 2nd turn, etc.). Lots of possibilities here.

Gluth - At the end of your turn, summon a 2/2 zombie chow. At the beginning of your turn, consume all zombie chow's and gain +2/+2 for each consumed. Puts it on your opponent to take it out or buff your fat. Sort of a mix between Hogger and Gruul.

Thaddius - No clue how to add polarizing effects while keeping it simple. Maybe something like a crazed alchemist that hits all minions instead.

Noth - Battlecry: Summon two 0/4 bone piles. Bone Pile: At the end of your turn, remove 1 HP and summon a 1/1 plague warrior. Basically comes with 2 watered down imp masters.

Gothik - Cult Master meets Redemption. When a friendly non-undead minion dies, return it to the board with undead modifier. Allows any minion to possibly have synergy with an undead themed deck while keeping only 1 resurrect per living card. No effect on existing undead. Massive potential given the right board state.

Sapphiron - 8/8 like the other dragons but with undead type. Battlecry: Freeze targeted enemy minion and destroy all non-adjacent enemy minions. Can kill anything but only minions 4 through 7. Kind of a reverse explosive shot. Opponent would have at max 1 frozen and 2 unfrozen minions on his turn. Worst case it's a frost elemental battlecry attached to an even fatter minion. Best case is a partial board clear with options. An awkward ability and sort of complicated for hearthstone but I really wanted to include the frost bomb somehow so there it is.

Kel'thuzad - Buffed archmage with frost nova + arcane explosion battlecry. Something like 4/9. Spell Damage +2. Battlecry: Freeze all enemy characters and deal 1 damage to them. Could replace the spell damage with summoning a random undead each turn: 2 1/1 skeletons, a 3/3 abom with taunt but no explosion, or a 2/4 caster with Spell Damage +1.

Mr. Bigglesworth - 3 mana - 1/1 - Deathrattle: Place a random undead minion from your deck into play. Undead version of Captain's Parrot / Sense Demons. Different from those by being a deathrattle and by actually playing the minion instead of just putting it to hand. Might be OP if included in decks with only a few powerful undead. Maybe instead, make the deathrattle like Animal Companion and summon randomly 1 of 3 mid-power undead. Alternatively, it could summon like Bane of Doom or Power of the Horde where it summons even more randomly from a larger list.