As incredible as the spiders are, it'd be a damn shame if only Vanilla spiders could be so intelligent and dangerous, when many other spider mods exist (Special Mobs, Primitive Mobs, NetherEx, Natura, Abyssalcraft, and so on).

I'm not sure how this could be best accomplished. A quick and dirty way could be to literally duplicate the spider entity multiple times, give them the appropriate texture/stat adjustments/on-hit effects, and you'd be set.

Spiders with more advanced code, such as the PrimitiveMobs Spider family and the Abyssalcraft spider, might be more troublesome to easily "convert" to their 2.0 upgrades.

Would you be open to accepting pull requests that attempted to add support for the above mods via the method I listed, or would you prefer that be an entirely separate fork of your mod?

I'm not sure how this could be best accomplished. A quick and dirty way could be to literally duplicate the spider entity multiple times, give them the appropriate texture/stat adjustments/on-hit effects, and you'd be set.

Not sure what you mean by that. Do you mean duplicating the other mod's spider class and modifying it to work with this mod and replacing the entities on spawn? That could work, but there are several things to consider:

1. the other mod's license must be compatible with LGPL (e.g. NetherEx with GPL is not), if the mod isn't ARR this could be circumvented by making a seperate addon mod with a compatible license
2. if the mod has special interactions with the mod's spider entity then doing it this way might break those interactions
3. it's a mess to maintain because you need to have a ton of duplicate code

I meant, for example, SpecialMobs' spiders have some unique features, but they are largely just "spider variants" with different base stats, textures, and on-hit effects. Rather than using any sort of Special Mobs code, one might instead just duplicate the necessary parts of Spider 2.0's spider entity, register it as a new entity under a new name, and then modify any necessary stats (movement speed, armor, jump height, and melee attack inflicting weakness, etc).

Specifically to avoid any issues of trying to reconcile the other mods' spider classes with Spider 2.0 code.

As you pointed out, any special interactions with those mods would likely not work, so Erebus, Primitive Mobs and Abyssalcraft for sure would require a less crude implementation to work.