What kind of rule changes have you folks tried at your tables, and how have they worked out for your games? Good? Bad?
Two of the houserules I implement for every campaign I run:
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No multiclassing until after 5th level, and no further multiclassing unless you have at least 5 levels in all your existing classes. I do this for two reasons, the first being to ensure that every character has access to extra attack/third level spells and slots/some other equivalent before they start dipping elsewhere, and to keep the munchkins at my table from taking multiple 1-3 level dips into classes just to set up a niche wombo combo. Even then, I'm pretty stringent on what I'll allow from a storytelling perspective - I want to know what motivates your Paladin to dip into Warlock besides getting to use CHA for attack and damage modifiers.
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Instead of an ASI or a Feat, every ASI level gives a +1 and a feat. My players and I like this rule because it allows them to pick something fun at those levels instead of feeling obligated to dump straight into the primary stat, and encourages grabbing those fun half-feats like Actor or Linguist that would otherwise go by the wayside.
I've got a few.
1: Massive damage cannot result in instant death. With the exception of fall damage, I can't see a reason that I'd ever want to do this to the players, and even with fall damage, players become unlikely to die from this as soon as they have an average of 35 hitpoints. If I have traps or monsters dealing enough damage to kill instantly, then I've misbalanced my game.
2: Similarly, monsters can't crit, however some monsters will now have new features that allow them to crit, some of which may have expanded crit ranges or inflict a condition on a crit instead, although you can presume a monster can't crit by default.
3: There is no heroic inspiration at all. PCs are already powerful and features like bardic inspiration or the help action already allow PCs to reward eachother.
However I'm interested in an idea which is players just voluntarily taking "momentum tokens" any time they feel like the flow of the session isn't in their favour, this could be from a bad spat of missing or fromcthe DM disallowing a broken combo or anything really. Players can then communally spend the tokens to increment a d6 in the middle of the table (1 token to put it down on the table on 1, 2 tokens to increment it to 2, etc to a max of 6), whenever any creature makes an attack roll or uncontested ability check, they add the value on the die to their roll, however if they're a PC, they add it to all 20 rolls. This means all creatures become more likely to cause their stuff to happen, basically. The big downside is that players can just award themselves tokens at their own volition, which is easily abusable, plus they may get used to a high momentum in a session where they have a +5 or +6 and then feel that the next session has less momentum (the 6d resets by session), and award themselves more tokens. Also if they use the tokens as frustration tokens, it may hurt my feelings. But I like the idea so I continue to workshop it.
4: I use the 2024 rules for exhaustion, unless via the sickening radiance spell.
5: If you are hit by an attack while in death saving throws, you don't automatically fail any, but instead instantly make another death saving throw, still failing on 1-9 but not gaining a success on 10-20. This actually increases tension because I'm more keen to actually hit PCs when down.
Those are the big changes, I do have a few other though,
I don't allow any of the "conjure x" spells, and generally wanna look over any features that add creatures to combat to save time in combat. The summon spells in Tasha's are good, as are most class feature that add creatures to combat, and find familiar.
I don't allow silvery barbs. Not just do I feel it's too strong, I also feel that it's flavourless and entirely interested in altering mechanics that are an abstraction of the storytelling.
I don't allow the lucky feat, however I have a custom feat called borrowed luck, it works the same but if you are reduced to 0 hitpoints, you instantly fail a number of death saves equal to the number of luck points you've spent that long rest.
I have a few bits of race lore such as much fewer races having darkvision but dwarves having access to the light cantrip (and other minor benefits) and a few minor stuff like that, nothing too exciting there.
I actually don't mind killing PCs with some of the above here - not saying that I seek out opportunities, but I've played with squishy casters who are bold and/or dumb enough to wade into the enemy's back line to take advantage of short range AoEs like Burning Hands and Thunderwave, and you better believe their response is to beat the ever-loving shit out of that caster so they don't get up and do it again. And past the opportunities that cocksure players give you, it is 100% okay if a character dies, even one that a player has invested in - adventuring is dangerous and combat is especially brutal; the dragon's not going to reposition themselves to exclude a downed PC from their breath attack, the vampire's not going to pass up an easy meal from an incapacitated caster. If your games are going to be impactful and climactic, the stakes need to be real, and you can't pull any punches.
But there is an important caveat in all this - what's not okay is trivializing PC deaths, whether they died through pure chance, or wildly unbalanced encounters that end in TPKs, because that (especially the latter) ruins games and creates players who invest nothing in their characters, or worse, start to see everything as a numbers game and work to build the murderhoboiest character they possibly can. If a PC dies, it needs to be a scene. After combat's over, make a point of narrating the aftermath. Give the PC last rites, have the surviving members of the party talk about their favorite moments. Some of the best, most heart-wrenching sessions I've run are the ones where a character dies.