When the PC's do something dangerous, ask them, "Do you want to push your luck?"
If they say no. Resolve the dangerous thing as you would do normally. If they say yes, pass them a Luck Token. (I'll explain what that is a little later, but its good)
Now choose one of the following things to make the dangerous situation potentially even more dangerous or complicated.
- Ask the Players to roll to see if there is a random encounter.
- Make the PC's involved pass an additional test or take damage.
- Have every PC involved roll a Usage Die.
- Give them Disadvantage on the test they're about to take.
- Split the party.
- Disarm the PC's involved.
- The next reaction roll is rolled with 3d6 (instead of 2d6)
- The next Initiative test will be with Disadvantage.
I'll likely finesse and add to that list. I want these things to be adaptable to any situation the PC's find themselves in (Social conflict, Exploration, Combat, etc) and have distinct mechanical hooks ie, they link to something that TBH does mechanically. Still some testing/work to do.
These are shared between the whole group, and can be used at any stage to automatically succeed at any test.
Additional thoughts - I think that you should use this sparingly, ideally I don't want the players having more than 1 or 2 Luck Tokens.Otherwise it would be too easy for them to just steamroller those really important moments. If its a scarce resource, tough decisions will have to be made when and where to use it. I'd probably start the group with one Luck Token, then once used, start thinking about offering them an opportunity to get it back.
Also consider when you offer to push their luck - I'd recommend dangerous situations that aren't life or death, but have the potential to really ramp-up the tension if they were to get more complicated. Climbing ropes, Balancing over thing beams, Sneaking ahead to scout and orc camp etc.
Anyways, that's it for now.