Ditch the proficiency bonus. Instead, each skill should have a list of Basic and Advanced applications.
Advanced: If you're trained in the skill, you can make a check to attempt these tasks. If you aren't trained, you can't even try.
Basic: Anyone can attempt these tasks. If you're trained in the skill, you succeed automatically. (In an opposed check, if you and your opponent would both succeed automatically, the successes cancel out and you both roll as normal.)
So, for example, Stealth might have "move silently" and "hide behind cover" as Basic tasks, and "move through shadows" as an Advanced task. Meanwhile, "notice a stealthy person" is a Basic Perception task. Let's say you have an infiltrator trying to sneak someplace and a sentry trying to spot the infiltrator.
If neither is trained, the infiltrator can attempt to move silently or hide behind cover, resulting in a contest of Wisdom vs. Dexterity. The infiltrator can't even try to move through shadows since this is an Advanced task.
If the infiltrator is trained and the sentry isn't, the infiltrator can move silently or hide behind cover and the sentry will never spot her. If she tries to move through shadows, it results in a contest of Wisdom vs. Dexterity.
If both are trained, the infiltrator can attempt to move silently or hide behind cover. This is a Basic task, which would normally mean auto-success, but the sentry also has an auto-success to spot her. The auto-successes cancel out and the two must wage a contest of Wisdom vs. Dexterity. The infiltrator will always fail if she tries to move through shadows, because this is an Advanced task (she has to roll) versus a Basic task for the sentry (automatic success).
If the sentry is trained and the infiltrator isn't, the infiltrator can never sneak past.
Advanced: If you're trained in the skill, you can make a check to attempt these tasks. If you aren't trained, you can't even try.
Basic: Anyone can attempt these tasks. If you're trained in the skill, you succeed automatically. (In an opposed check, if you and your opponent would both succeed automatically, the successes cancel out and you both roll as normal.)
So, for example, Stealth might have "move silently" and "hide behind cover" as Basic tasks, and "move through shadows" as an Advanced task. Meanwhile, "notice a stealthy person" is a Basic Perception task. Let's say you have an infiltrator trying to sneak someplace and a sentry trying to spot the infiltrator.
If neither is trained, the infiltrator can attempt to move silently or hide behind cover, resulting in a contest of Wisdom vs. Dexterity. The infiltrator can't even try to move through shadows since this is an Advanced task.
If the infiltrator is trained and the sentry isn't, the infiltrator can move silently or hide behind cover and the sentry will never spot her. If she tries to move through shadows, it results in a contest of Wisdom vs. Dexterity.
If both are trained, the infiltrator can attempt to move silently or hide behind cover. This is a Basic task, which would normally mean auto-success, but the sentry also has an auto-success to spot her. The auto-successes cancel out and the two must wage a contest of Wisdom vs. Dexterity. The infiltrator will always fail if she tries to move through shadows, because this is an Advanced task (she has to roll) versus a Basic task for the sentry (automatic success).
If the sentry is trained and the infiltrator isn't, the infiltrator can never sneak past.
