I was thinking there should be guilds for every skill in-game, maybe mostly along the same vein as the mining guild where there are extra resources and that's about it. But thinking about adding a few scattered and potentially problematic guilds seemed like it wouldn't be worth the effort, so instead I've come up with a new guild... the Skiller's Guild! Basically would have requirements for every skill represented (somewhere between 60 and 68 each), and represent each of the skills not yet represented by a guild of some sort: woodcut, firemaking, fletching, herblaw, thieving, and agility.
LEVEL ONE would act as a Woodcut/Firemaking/Fletching guild with plenty of rarer trees such as yews and magic trees, plus a place to pick and possibly spin flax, as well as a wide open space to light fires. Have there be a med-level axe spawn and a knife spawn. This would be a large, ground-level, fenced-in area with a ladder or cave entrance somewhere in it. Thinking the already partially fenced area that occupies the space southeast of the wood yard and northwest of the digsite would work out. Or possibly over west of the gnome stronghold.
LEVEL TWO would be a dungeon that encompasses Herblaw, thieving, and agility. It would be a series of rooms/caves linked by agility obstacles not unlike the yanille agility dungeon, but each room would have a few mobs specific to each skill. For herblaw it could contain chaos druids for herbs, rooms with red spiders and red dragons and hobbs and black unicorns for spider eggs/whiteberries/limps/snapes/horns, as well as a room with a mortar and pestle, a fountain, or a merchant to buy vials. For thieving, put more than one knight in literally any goddamn area, maybe create a few unique chests and traps, lockpick spawns, etc.
Could consider adding NPCs with skill-specific stores to buy rarer secondary ingredients at a premium (instead of spawns, maybe), or pre-made potions.
If this seems too overpowered the catch would be that generally it is somewhat far from any convenient banks. If that makes it too useless, put a banker after a very high lvl, easily failed agility shortcut so it is difficult and risky to reach. I think this would be a bonus to people who have trained high lvls in all skills, and also give people training something to aspire to.
Feel free to tweak this idea and make it more realistic.