Extending the idea in the “Initiative” post.
In a non-level game add an “experience” attribute that measures the number of seriously life-threatening incidents a character has had and overcome.
Say the range is 0 to 12 and a character starts with 0-4 depending on whether the game implies earlier life experiences e.g. in Cthulhu an ex-gangster might start with 3, an ex-cop 2, ex-military from 0 to 4 depending on if they were in a war or not, and most characters 0.
After surviving a seriously life-threatening incident a character would try to roll above their current experience level on 2d6 to increase the rating by 1 (so experience would increase rapidly early on followed by diminishing returns). This experience rating would be the basis of their initiative rating (modified by Int, Agi, special talents etc.
So a character with an experience rating of 3 would need to roll above 4+ to increase the rating to 4.
Each such incident would also require a sanity check where their experience rating could be their save i.e. the same character with an experience rating of 3 would need to roll 3 or less on 2d6 to avoid a sanity check. A grizzled veteran with an experience rating of 8 would only need to roll an 8 or less.
Characters who start with experience may have to make some insanity checks at the start.
Life-threatening situations wouldn’t necessarily have to be combat. They could be anything which could lead people to crack up.
For example I am going to use the idea of some solar systems having a kind of surrounding gravity sink in my Traveller game. Jumps are supposed to take a week but jumps into systems surrounded by a gravity sink take an extra week when the navigator knows they are there in advance but 1-6 extra weeks if they don’t. So the first explorers into such a system will be in jump space for an extra 1-6 weeks not knowing how long or if they will ever get out. That might give an experience roll for each extra week providing a strong boost to experience but with the possibility of some serious mental damage as well – possibly even an impromptu encounter trying to sedate one or more of the crew.
So a starting character going through this could potentially have an experience of 6 (out of 12) in the first hour.
Incidents would have a threshold rating where they no longer counted for experience e.g. a standard human vs human firefight might have a threshold of 8 so it doesn’t effect a character who already has an experience of 8. A fight against a dog-sized bug might have a rating of a 2 if in the open in daylight against 1 of them but against a horde of them (+2) in the dark (+2) in cramped cave underground (+2) then the threshold might be 8.
A player would only roll for incidents rated higher than their experience rating.
Failed experience rolls would have some kind of cumulative insanity effect.
For example create an insanity table with rows from 0 to 20
– 0 to 8 are blank
– 9 says “likes a drink, occasionally needs one.”
-10 says “occasional nightmares.”
-11 says “Starts smoking.”
-20 Full Arkham asylum treatment.
Then every time a character fails an experience check they add 1-3 to their insanity roll taking whatever penalty they get – which are mostly just flavor until they reach the top end of the table where the penalties become severe.
It needs more work but I think this is a good idea in essence. What it would mean is your starting character would either a) be much tougher or b) have started to crack up completely, just getting through their first dungeon / battle.