I might revise this just a bit later so I can suggest a change to the Spiders' relationship with Orcs and players. They should definitely be a lot more hostile, although I don't think we should completely remove the ability for players to befriend Spiders.
Also a relevant quote for Shelob's appearance, which is kind of what the Gloom-weavers should look like, but not as large and with smaller features.
"Most like a spider she was, but huger than the great hunting beasts, and more terrible than they because of the evil purpose in her remorseless eyes. Those same eyes that he had thought daunted and defeated, there they were lit with a fell light again, clustering in her out-thrust head. Great horns she had, and behind her short stalk-like neck was her huge swollen body, a vast bloated bag, swaying and sagging between her legs; its great bulk was black, blotched with livid marks, but the belly underneath was pale and luminous and gave forth a stench. Her legs were bent, with great knobbed joints high above her back, and hairs that stuck out like steel spines, and at each leg's end there was a claw."