As part of a commentary on another thread, I provide the following suggestion. The basis of this idea is actually to respond to a comment and complaint about people hiding behind objects at night and attacking players being used as a reason to use the X-ray and Lighthack functions of the Gilbertus (And presumably other) clients.
Along with transparent objects (having them fade when you walk behind an object that obscures your vision), and having creatures/people behind objects be outlined for other players, (Even in the dark) I suggest the following.\
(Note: My Search-Fu is terrible, and while I did search, I didn't find anything relevant)
Stealth: If someone has sufficient stealth and intelligence one can remain stationary without being seen (provided they have stealth move on), provided they can overcome the other individuals Exploration & Perception.
Move Silently: This would be a passive Stealth & Agility based ability that allows one to move while in stealth, so long as they have a higher Stealth & Agility (modified by terrain and a random element) than their opponents Perception & Exploration they would be able to move without being revealed to another player. However, rather than merely showing the player if a failed check was made, a ping ring (of a shade other then red to indicate it's purpose) would be provided pointing in the direction of the stealthy player. Additionally, the player who noticed the moving players gets a temporary scaling buff against that player that starts with "snapped twig", and with every further success against the stealthing player gets an improvement. (Snapped Twig, Rustling in Underbrush, Eyes in the dark, and "Bull in china store").
Negative Modifiers can apply to this skill check, including:
1. Moving at speeds higher than a crawl.
2. Wearing non-leather armor (IE - Anything but a Cut-throat Cuirass)
3. Time of day for Stealth (Though perhaps the opposite should be true, night provides a bonus)
4. Weight carried? (If that ever gets added)
Neither of these abilities are available if the person in question is carrying a light source, or carrying a liftable.