So, I was thinking heavily on the matter awhile back, and I realized that when there are only one or two players each playing one hero, the group can quickly be overwhelmed with Spawns, and the overlord almost always wins, so I was thinking on some homebrewed rules to fix this problem, preferably something relatively simple.
Maximum Spawn Limit: Whenever the overlord plays a Spawn card, they put it down in front of them. When a monster represented by the spawn card dies, they set it atop the card, until all the monsters made by the spawn card die, at which point the overlord puts the spawn card in the discard pile, though the specific creature need not be spawned by that card. So if the Overlord had two Beastman War Parties in play, and the heroes killed two beastmen from one card, and one Master beastman from the other, they would all be put on one card, and that card would be discarded. The Overlord may only ever have as many spawns played as there are players. When playing with one hero, the overlord cannot play another spawn card directly after a spawn card has been discarded in this way. Creatures need not be spawned by a card to be affected by this rule.
This one rule should remedy the problem, hopefully, and I hope someone can get some playtesting in with it. Was also thinking of having spawn cards spawn less creatures for less heros. Anywho, could I get a bit of feedback on it? I will be editing this post with any good suggestions of homebrew rules to fix play for 1 or 2 heroes.