I'm sure that spiders are already nightmare fuel for most players, however as there are already wolf spiders and spiderlings in the game I wanted to make a suggestion.
Wolf spiders in real life are known for carrying their young on their backs. If it's not too complicated of a mechanic, I feel that it could make for a good "broodmother" boss to have a mother wolf spider with her babies in tow. Two options could work for this.
1.) When the mother dies, a swarm of spiderlings are released (10 ish to reduce lag despite them usually being 100s).
2.) When the mother takes damage, 1-3 spiderlings pop off her back.
Keep up the great work Obsidian!