Arachnocraft

-Trapdoor Spiders spawn in trapped divots in the ground, and try to kill you if you fall in.  They drop 0-2 poison sacs and can be "milked" for a bucket of poison

-Powerful Skeletal Spiders that spawn deep in the nether and the Spider's Den, they drop 1-3 whole bone blocks!

-Phase Spiders, which have the ability to warp and spawn between all dimensions, and drop Unstable String, charged with unstable dimensional energy.

-Longlegs, which spawn in cold biomes and drop their own legs used to create the rod of the staff used to enter the Spider's Den.

-Zombie spiders spawn exclusively in the Spider's Den, and have exposed brains.

-Tarantulas, which are slow, powerful spiders whose exquisite fur can be used to create thick armor.

-Spitter Spiders, which spit sticky webs and you and their spinnerets can be used to create the Web Launcher.

-Arachonstructs, which can be summoned by using a Spidric Key on stone or an iron block and attack hostile mobs.

-Stinger Spiders, which are definitely spiders and not any other arachnid, spawn in deserts and drop poison sacs used to poison a diamond sword.  They will give you nausea if you get too close.

-Arachnid Merchants, which are part spider, spawn in underground houses. Trade them a gold block for a Blood Diamond, which you can use to make the mod's custom crafting mechanism.

-Spider Crabs (suggested by Boolyman), which are maybe spiders, spawn along rivers, beaches, and the ocean and are neutral mobs.

-Slime Spiders, which are pretty slimy and spawn in swamps.  They split into 3 small slimes upon death.

-Many more spider variants

-More structures

-Better animations and spawning

-Arachnomancy

-Drow elf ruins

-A new boss- Grandpa Longlegs!

-Even... rideable spiders!?!?