Stealth

One possibility might work thus:

• You begin by gaining Stealth. You get a certain number of points.
• You may take actions. Each action has a certain cost.
  • Observing silently costs nothing. Good for spying.
  • Moving slowly costs very little.
  • Stealing an unattended item has a modest cost.
  • Ambushing an unaware target has a high cost.
  • Moving quickly also has a high cost.

Should you run out of Stealth points, you will be noticed. You can lie low for a time to regain Stealth points. However, this not being a videogame, alert guards might not rest easy for hours or even days if they spot a burglar but never catch him, nor see him leave.

Ambushing targets while in Stealth should work quite differently from sneak attacking in combat. Quite simply, a rogue should be outstanding at this. It should require a great amount of preparation, timing, and even luck, but it should be possible to assassinate targets through superiority of Stealth alone.

Randomness can be injected into this system by making small adjustments to the player's Stealth score without his knowledge. He still knows his own score, but it isn't entirely accurate.