how do you make lethality fun in TTRPGs?
As a gamemaster, you've probably run into this problem. Your players mess up, or maybe you made a fairly unbalanced encounter, or maybe it's pure dumb luck, but one of your players has to die. Naturally, the person playing this character is going to be fairly upset. After all, they've invested time into this game, immersed themselves in this character, dedicated thought and energy to who they are and what they do, not to mention put together the character sheet, which in a lot of systems can be a chore and a half.
What do you do, as a tabletop game designer, to mediate this issue? In some systems, they make the concept of death a far off thing that only happens in the most dire of circumstances, and something that can be reversed. In some, they allow proverbial second and third chances, such as death saves in Dungeons and Dragons or Pathfinder. Some systems just forgo the concept entirely, providing a different host of negative consequences for failure. Some games don't even give thought to this issue, bluntly presenting you with the idea that your character can die and this should be expected, while not really softening that blow in any way, or reducing the personal involvement you have to have in a character.
These examples would lead you to the conclusion that death is somewhat of a necessary evil in tabletop games, something that is unfortunate and not too fun but that needs to exist. I disagree.
To solve this particular issue, instead of looking to other tabletop games, I look to video games. In video games, specifically those with "permadeath", the tradeoff is that when you do come back as a new person, it requires very little from you as a player, starting you at square one and letting you go free. Roguelites are a fantastic example of this: you invest time and thought into a build/run, but yet death is omnipresent and something that must constantly be outrun or avoided. This idea, that death can come at any time, anywhere, leads to very dynamic gameplay situations with built-in, real stakes.
To translate this philosophy to EX-OP, my TTRPG system, I recognized two major systems that would need to be molded to fit it: character creation, and moment-to-moment combat.
Character creation need to quick, snappy, and intuition-friendly. Something that can be memorized, and that after a few games you should be able to create most of a character in your head. The Proficiencies system is very conducive to this, as characters are assembled piecemeal through a "bucket of legos" list of skills and abilities. If you want to play a stealthy sniper who has a pension for drinking and general merriment, you would approach this in game terms by selecting the Major Proficiencies of Spotter and Infiltrator, along with the Minor Proficiencies of Sniping and Revelry. This is a very simple system that allows you to put together a character in a very short time. This means building a character is easy, quick, and is more focused on what they do rather than who they are, allowing that to develop naturally.
Moment-to-moment combat is a harder thing to balance. Excess lethality is a double edged sword. On the one hand, enemies are easy to kill, and combat is more satisfying. On the other, players can die almost as easily as enemies, depending on where they're hit. In order to balance these things together, combat needs to be tactical and with as much player agency as reasonably possible. There should be means for skilled players to make it through entire encounters with hardly a scratch, and at the same time, means for players to die if they do something stupid or poorly planned. Players need to have deep toolbelts, and a lot of actions. For example, in EX-OP, a given player with 6 REFLEX and some decently leveled Proficiencies may be able to take up to seven actions on and off of their turn. They get four Action Points, where one is spent to do a Half-Action and two are spent to do a Full Action, and at 6 REFLEX they get three Reaction Points, which allow you to perform Half-Actions in response to being attacked. In addition, movement is handled as it's own system completely, meaning every action you take will be spent on tangible maneuvers.
This means that players on their turn have capacity for complex, involved maneuvers, while not sacrificing their freedom to position themselves advantageously. Having this number of moves means that players should always have a move to make, and getting trapped between a rock and a hard place is rare. Thus, when they die, they will most likely be able to recognize what they could have done to avoid it.
My core point here is a bit counter-intuitive: In order to make lethality fun, you have to give your players enough agency for death to be their fault. In EX-OP, you don't die because some giant enemy happened to crit you twice in a row, you die because you were in a bad position and used your actions poorly. While this might seem like handing off the responsibility to the players, the end result is that players consider their actions with far more weight and care. They see their characters as independent entities which they have sole responsibility over. This creates stakes and drama, without much if any input from the GM or the game itself.
TL;DR: In order for you to have a fun game with high risk of death, you have to make the death of the player the fault of the player.