Title says it all. I don't really play games, but I often think about playing games, and these are some ideas that have crossed my mind in those times. I haven't experimented with any of them or fleshed them out into tables, and you can probably find a half-dozen other blogs that go into much better and greater detail about any particular one of the concepts I play around with, but it's my show damn it.
Combat is Terrifying: When the characters aren't starting off as experienced combatants, or when your game is intended to be low-combat but still dangerous, consider emphasizing how frightening combat and both sides of violence really are.
Force attacking PCs to make wisdom/will/vs paralysis/etc. saves before they even roll to hit AC or cast an overtly harmful spell, because sudden and decisive violence is not a reflex most people exercise regularly. Failure means they hesitate or even freeze. And if they are themselves the target of an attack, a certain amount of damage or debilitation suffered should call for a save vs. fear because holy crap that hurt and that's your blood!
The PCs have to succeed (or maybe fail) a certain number of saves or endure enough rounds of combat overall to get desensitized to inflicting and being subjected to violence, and thus stop having to roll. Martial characters like fighters might get a bonus to these "ruthlessness" saves according to their to-hit bonus, or they might automatically desensitize at a certain level. And even when a character has reached that point, consider the mental toll it takes, and the stress it causes long-term.
Make sure enemies that aren't all battle-hardened or utterly single-minded automatons also deal with some of this in the form of morale checks for major setbacks or just for being green recruits. Remind players of the NPCs' human(oid)ity, and offer avenues for diplomacy or escape often.
Obviously don't do something like this without the whole group's consent beforehand. As I write this, I realize it might cleave too close to PTSD, which is very real for some people.
Journey and Destination: Instead of leveling up according to the number and lethality of the things you kill, the value of stuff you steal, or the narrative beats of a GM with more important things to do than balance an EXP checkbook, you gain experience by the literal milestone. This is most useful when travel and discovery is the central thrust of the adventure. Leveling up can happen overnight while camped, or maybe take the form of sudden insights of "road wisdom" as they leave an area.
Requiring that the party keeps moving in order to keep (mechanically) growing discourages them from settling down in one place for too long and becoming stale, stagnant, and/or feudal tyrants. Either give XP per the kilomile or other unit of measure traveled, or grant it in larger chunks depending on the length or difficulty of the paths the party chooses (or is forced) to take. In the event that they're moving over trackless wastes or acting as the trailblazers, consider awarding XP per region/landmark discovered, or map hex explored. Retreading old territory doesn't grant any experience unless they make a discovery they missed before.
If you need to positively incentivize them sticking around long enough to actually do stuff in every area instead of just scratching the surface like highly armed tourists, grant bonus experience for fully delving into the area's subsites, like ruins or a cave system, so that it guides them toward quests or bonus objectives. Finding the magma antechamber gets them the experience, but solving the fire lich's chronic loneliness gives that experience satisfying meaning. Or to give a negative incentive, slap the party with a stacking road-weariness debuff if they're being too shallow or moving on too quickly.
Communal Leveling: You're only as strong as the community you help cultivate and protect. PCs don't directly power up from doing or experiencing anything out in the field. Nobody levels up. Rather, they bring their experiences and resources back home to their base (or their caravan/nomadic camp if you want to use this with the above travel rules) to help it grow and prosper.
Different specialized buildings can be constructed, NPCs are attracted to the community, and existing relationships strengthen over time. From these, the PCs will gain their information, equipment, upgrades, training, etc. that were previously gated or inaccessible. And if the PCs ever start to neglect their community or take its services for granted, they risk misfortune befalling the place and/or its inhabitants, and making those services unavailable again.
And unless it's part of a big, endgame culmination of plot and progress, don't let the characters outright take control of the community. Doing something like taking public office and greater responsibility in recognition of their contributions is fine, but make sure they remain accountable to their fellows- fantasy is one of the few places where we get to make that happen without a lot of canvassing and/or revolution, after all.
The PCs are important, even vital organs of the whole, but this is not another stronghold for them to rule over. These are friends, family, and neighbors.
Moar Depletion Dice!: Instead of just making a sword break on a Nat 1, have the blade chip or bend first so that it's still usable, but not as effective until it gets a serious whetstone'ing. To-hit can take a penalty, but also that d8 damage drops to a d6. If the game uses a hit dice recovery during rest mechanic, maybe suffering some critical hits slashes those dice until the PCs manage a very long rest. Interrupted concentration or fizzled spells reduce their numerical effects by dice sizes, too. And all of these stack, to create an experience of gradually rising desperation as abilities and resources are depleted.
Basically anything that isn't a d20 has the potential to get reduced through injury or wear and tear. An entire mishap table can be constructed from these, offering a less permanent and gruesome alternative to the detailed maiming of some injury tables- or a complementary system to integrate into them, if your players are Hidetaka Miyazaki-level masochists.
Unless something obvious escapes me, this rule leaves most non-variable utility, save-or-die, or save-or-suck spells and effects unaffected, since there are no dice involved to deplete. It can still impact other facets of the characters or creatures who use those types of abilities enough to not be an all-the-time issue, but keep in mind that specialist wizards and their ilk might stand out as even stronger than usual.
Natural; Not Divine: The source of power that rangers, shamans, druids, et al draw their magic from is categorized as primal rather than divine.
... That's basically it, I just want it treated as unique and special.
I had forgotten all about the D&D 4E power categories until I started reading Keith Baker's old Eberron backlog and found that he has continued to apply the primal distinction to druids in his version of Eberron, not as a mechanical label but to help characterize a very distinct set of animistic practices and traditions as separate from the usual monolatry and organized religion you find in a D&D-inspired world. I just find it neat and wanted to share that.