Glaurung pretty much wants a non-martial art dependent assassin-like guild...
Sir the Master Merchant
For Pricing Information - "finger sir"
What would be interesting, though not quite original, would be a rogue style
guild. They could be given general melee combat skills, along with the
aforementioned backstab/strangle. I would think stealth would be an important
aspect of a rogue, so they could have sneak/hiding/darkness etc, and maybe
skills for setting and disarming (read: detecting) traps. This is quite
obviously a very under-developed idea, but you have to start somewhere.