But aren't.

The title pretty much says it all.   Whether it's something that used to be commonplace but now isn't or something that has always been very niche, what's a game mechanic that you wish was more widely adopted by the bigger more mainstream genres and games?

A couple years ago, I would have said detailed (h)exploration mechanics when I was first introduced to them with Forbidden Lands; in the years since with the growth of the OSR reintroducing it to quasi-d&d as well as some variant incorporated into official D&D with the 2024 version, I don't think it qualifies anymore as I think the mechanic has tipped over the critical mass line.   I'll instead go with usage die mechanics for consumables (also introduced to me with Forbidden Lands but obviously exists in other games).

I can't think of one.  Game mechanics don't exist in a vacuum.  They need to work well with the other mechanics in that particular game.  In fact, I think falling in love with a mechanic is one of the big pitfalls for a designer. 

That said, if you want to talk about relatively unexplored design space (regardless of how often or not it can be used well in a system), then I'd nominate "moderate chunks of archetype or capability".  There's no good name for it generically, but in D&D terms you can think bigger than a feat or skill, smaller than a class.  The 5E class options are about the right size, but don't qualify for this because they are just big options that the class gets instead of independent things.  So in other words, it needs to get picked like feats but work more like a sub class.

The exact details, of course, are going to depend a lot on how the game otherwise works. 

-Slot-based inventory/encumbrance. I think it's the best compromise between realism and gameplay convenience. Probably the only reason it hasn't caught on is that 90%+ gamers are already used to ignoring encumbrance entirely.

-Simultaneous attack and defense in melee. To me, this is just obviously how combat should work, but I think I've only seen it in Warlock! and the system Zalman posted recently.

-Armor penetration rolls/armor saves. I feel less strongly about this one, but it makes more sense to me than armor soak or armor folded into defense score.

-Chase/pursuit rules. They're starting to be more common now, but they were a glaring omission in most mainstream games for a long time.

-mix-and-match class/profession systems a la Warhammer Fantasy Roleplay or Shadow of the Demon Lord. Not a good fit for every game, but it's a great system and far superior to D&D multiclassing.

Usage Dice.
They were designed to be used for arrows and consumables but forget that nonsense, the concept is better used for things like Sanity, replacement for spell slots, or for hireling morale. You could even use usage Die for random encounter tables, When the Usage Die drops you drop to a different encounter table, maybe more dangerous, maybe the alarms went off or the volcano is starting to blow or something.

1-vs-1 dueling.

Especially in Star Wars type games.

My preferred Star Wars game is the simplified Mini-Six Bare Bones game (which is free FYI), and I found a plasma sword (lightsaber) fight is typically over in one hit on turn one or two.

Then I found Dueling Blades by Griffon Publishing (also free) that completely changed duels to a drawn out and interesting fight that involved movement as much as dice rolling. It's great.

Recently I bought Space Pulp RPG for Everywhen/Barbarians of Lemuria.  It also has dueling rules built right into the game.

I'd like to see more games where injuries are less common (easier to avoid being hit/damaged) but where the injuries are more meaningful and harder to remove (e.g., you can't just take 8 hours to sleep off nearly being killed).

I really like the ability to push yourself at the expense of some condition, like in Dragonbane. I also like the flashback mechanism from Blades in the Dark to account for times when your player knowledge isn't sufficient to plan for in-world things your PC would likely know - of course, it needs a cost associated with it, but it can make for interesting sessions.

Quote from: ForgottenF on March 28, 2025, 09:38:58 AM-Simultaneous attack and defense in melee. To me, this is just obviously how combat should work, but I think I've only seen it in Warlock! and the system Zalman posted recently.

What's the system that Zalman posted recently?

Simultaneous attack and defense is standard in Powered-by-the-Apocalypse (PbtA) systems, which are quite widespread. So this one is popular, just not in the OSR.

Quote from: Shteve on March 28, 2025, 12:37:41 PMI also like the flashback mechanism from Blades in the Dark to account for times when your player knowledge isn't sufficient to plan for in-world things your PC would like to know - of course, it needs a cost associated with it, but it can make for interesting sessions.

Quote from: ForgottenF on March 28, 2025, 09:38:58 AM-Simultaneous attack and defense in melee. To me, this is just obviously how combat should work, but I think I've only seen it in Warlock! and the system Zalman posted recently.

I think Pendragon has that sort of thing.

Quote from: jhkim on March 28, 2025, 12:53:39 PM

Quote from: ForgottenF on March 28, 2025, 09:38:58 AM-Simultaneous attack and defense in melee. To me, this is just obviously how combat should work, but I think I've only seen it in Warlock! and the system Zalman posted recently.

What's the system that Zalman posted recently?

Simultaneous attack and defense is standard in Powered-by-the-Apocalypse (PbtA) systems, which are quite widespread. So this one is popular, just not in the OSR.

https://www.therpgsite.com/design-development-and-gameplay/the-smor-system/

Is that a thing in PBTA? The only PBTA game I've played is Dungeon World, which either doesn't work that way, or my DM was running it wrong.

Quote from: Ruprecht on March 28, 2025, 12:58:14 PMI think Pendragon has that sort of thing.

Huh. I thought it just had the standard "parry" action you get in a lot of games from that lineage. I'll have to go check.

A single roll for attack and damage.

I agree on wilderness procedures. One thing I think should be more popular is "reverse random encounter rolls", i.e., instead of rolling 1d6 to see if you have an encounter TODAY, roll 1d6 to see HOW MANY DAYS until next encounter. This saves quite a lot of rolls.

(OTOH automation would be even better for that).

Quote from: ForgottenF on March 28, 2025, 01:10:44 PM

Quote from: jhkim on March 28, 2025, 12:53:39 PM

Quote from: ForgottenF on March 28, 2025, 09:38:58 AM-Simultaneous attack and defense in melee. To me, this is just obviously how combat should work, but I think I've only seen it in Warlock! and the system Zalman posted recently.

What's the system that Zalman posted recently?

Simultaneous attack and defense is standard in Powered-by-the-Apocalypse (PbtA) systems, which are quite widespread. So this one is popular, just not in the OSR.

https://www.therpgsite.com/design-development-and-gameplay/the-smor-system/

Is that a thing in PBTA? The only PBTA game I've played is Dungeon World, which either doesn't work that way, or my DM was running it wrong.

I think your DM was running it wrong. In Dungeon World, the DM never rolls for enemy attacks. (The DM never touches dice at all for resolution.) Instead, the player rolls his "Hack and Slash" Move. On success (10+), only the PC does damage. On a limited success (7-9), both sides deal damage. On failure, only the enemy deals damage. Here's the Dungeon World SRD

https://www.dungeonworldsrd.com/moves/

Quote from: ForgottenF on March 28, 2025, 01:10:44 PM

Quote from: Ruprecht on March 28, 2025, 12:58:14 PMI think Pendragon has that sort of thing.

Huh. I thought it just had the standard "parry" action you get in a lot of games from that lineage. I'll have to go check.

I just checked my Pendragon 1st ed, and melee is a contested roll so only one side can win and do damage. I don't know about later editions.

Quote from: Ruprecht on March 28, 2025, 10:34:05 AMUsage Dice.
They were designed to be used for arrows and consumables but forget that nonsense, the concept is better used for things like Sanity, replacement for spell slots, or for hireling morale. You could even use usage Die for random encounter tables, When the Usage Die drops you drop to a different encounter table, maybe more dangerous, maybe the alarms went off or the volcano is starting to blow or something.

Obviously I'm a fan of usage die mechanics as I suggested the same in the OP but I never thought about using them for encounters and other things.   Interesting...

Quote from: HappyDaze on March 28, 2025, 12:32:21 PMI'd like to see more games where injuries are less common (easier to avoid being hit/damaged) but where the injuries are more meaningful and harder to remove (e.g., you can't just take 8 hours to sleep off nearly being killed).

I think game designers are worried about death spirals that may result.  My most recent experience was with the Alien RPG by Free League last year where my heavy gunner took an unlucky wound early in the mini-campaign to the arm and was now just a pistol shooter.  Since it was a mini campaign and the healing time was beyond the scope of the campaign (the characters were on a "base is self destructing in t-minus... situation) so I was just mostly useless in my primary role for the rest of the sessions.  In that case, a d&d style "long rest" wouldn't have helped but the injury met your criteria in that it was quite meaningful and effectively impossible to remove... and it made the gameplay experience much worse for it unfortunately.