Initial changes recommended:
Attacking the Horde directly is no longer so simple - Before damage can be done to the Horde's library (Causing it to mill), first they must control no creatures with the capacity to block while untapped (IE, if they swung with three zombies, you need to kill those three before you can hit their deck, unless those three happen to be Walking Dead , which can't block under any circumstances. You just walk past them. Pacifism also lets you get around enemies).
"So how do I kill so many zombies?" you ask. It's simple - You go Hunting. On the Survivors' turn, they can Hunt tapped Horde creatures, effectively being allowed to use Prey Upon at-will for free, except is causes the Survivors' attacking creatures to tap. If the Horde controls any untapped creatures capable of blocking (Vigilance), those must be dealt with first.
If the Survivors would ever deal damage to the Horde's health (Red burn, Extort, you name it) while they still have creatures capable of blocking when untapped, instead that damage is dealt to one of the Horde's creatures. Whenever the Horde would deal damage to itself, it actual mills that number.
Some creatures are "Guarded" (Will be included in Horde-Specific Rules), meaning that they do not attack as normal, and that they can't be targeted with Hunting until the Horde controls no other Hunt-able creatures that aren't Guarded.
Survivor health shouldn't go UP as the number of survivors goes up, it should go DOWN. Why did the original rules make the health positively correlated to number of players? Makes it WAY too easy for 4 players. Nah, try these starting healths:
1 player: 60
2 players: 50
3 players: 45
4 players: 40
Further, the survivors shouldn't be allowed to ever half more than 20 over their starting health, to prevent life-hoarding.
Decks shouldn't be fixed at 100 cards with about 60% tokens. That makes little sense, is terrible to balance around, and is generally way too fast. There should be more horde monsters than player monsters. They're the HORDE for crying out loud. By standard rules, the Horde will be getting mobs about as fast as the survivors combined, except their stuff will be getting blocked, doombladed and otherwise taken out. My solution? Vary the number of cards and percent of tokens in the deck. The percent of tokens determines the average number of tokens played per turn. This can also be used to balance the differing base stats of the races - IE 1/1 Goblins would have a greater % of tokens than, say, 4/4 Giants. To start, I'll fix the number of non-tokens at 50, as all that really does is determine the number of turn-breaks the deck has.
Initial ideas for token-counts for various racial templates (TWEAKING NECESSARY):
1/1: 200 (Humans, Goblins, Kobolds, Elves, Slivers)
2/2: 150 (Zombies)
2/2+ability: 125 (Vampires)
3/3: 125 (Centaurs, Minotaurs, Golems)
4/4: 100 (Giants)
4/4+ability: 75 (Angels)
The Horde needs to ramp up with the players. When their Library is half gone or more, they begin taking double-draws (At the start of the turn, they draw until they draw a non-token twice instead of once).
Of course things will be tweaked between decks, and perhaps different Hordes could have their own rules. Also, each deck of mine will include specific rules for card abilities that aren't straightforward. For example...
HORDE SPECIFIC RULES:
You'll notice this deck has a Grave Titan as it's commander. How Commanders work in Horde is that once the Horde begins taking turns, it counts down a timer equal to the CMC of the commander until it plays the commander in addition to it's regular turn. Whenever the commander would be destroyed, exiled, tucked, or otherwise made useless, it instead returns to the command zone and is re-played in two turns. Commanders attack each turn, as normal.
See welknair zombie horde deck her: Super Horde Magic: Zombies