Lorespawn

• Mob farms get more complex, as a dark room won't be able to spawn much mobs. Renewable methods to gain the drops are planed, adding more depth to the concept.
• You don't have to spam torches as much. (If all mobs get changed). However mobs will be able to spawn even in torched places as long as there's a spawner block nearby
• Multiple ways to counter mob spawns. Adventuring, blocking off areas, breaking the spawners directly are all valid methods. 

Spiders begin their life cycle as egg sacks. You'll be able to find these blocks generated like ores underground. A spider might spawn from it when you make sound around it and convert into cobweb blocks.
Connected egg sacks will break all at once when they're broken. It'll explode into cobwebs and a bunch of baby spiders. Baby spiders try to get as far away from the player and each other. Once they've stayed around for some time, they'll find a suitable position to make a spider nest, and move into one.
Spider nests act similarly to egg sacks, spawning a full grown spider once a player is noticed. However, breaking one could be more dangerous than breaking a nest! Spider nests can generate in various places during worldgen.