This wouldn't work in some earlier Dragon Quest games:
* DQ2 and DQ6 have mandatory items sold in shops. (Well, in DQ2 you *could* get around this by leveling up high enough, but that wouldn't be much fun, and would make the later game easier than it would be if you hadn't don this.)
* DQ3 has one item in a shop that you need unless you raise a certain class to a certain high level and learn the spell that mimics the item, and the level it's learned at is of course somewhat randomized.
* DQ4 doesn't have any mandatory shop purchases IIRC, but I think Chapter 3 would not be much fun without the ability to use the shops. (Worth noting that Chapter 3 in DQ4 has random wandering shopkeeper encounters, a mechanic I've only otherwise seen in Demon's Winter.) Shin Megami Tensei: Nocturne does this on its hard difficulty setting. Of course, it's possible to create configurations that don't lead to meaingful gameplay. Like, in Civilization 3 for example:
* Solo play with no other civilizations, with Conquest Victory enabled; you win right away. (Note that disabling Conquest Victory does not disable Conquest Defeat, as there's no other sensible way for the game to handle the player's civilization being wiped out.)
* Degenerate custom maps. A map of all mountains is an easy victory, as the opponents will all disband their settlers (I assume their AI panicked looking for a place to build a city), while a map of all water will cause the game to fail to start, with an error to the effect of "can't place initial settlers". I've heard Shining Force 2 does something like this. The two middle difficulty settings increase enemy stats, but the hardest puts the enemies at their default stats, instead upgrading the enemy AI. I think time limits could work in a special time attack challenge mode, or perhaps in a game that's meant to be a racing game. (A time limit on a race would be better than a rubberbanding AI; in the Goron race in Zelda: Majora's Mask, I'm pretty sure my personal best time was a loss, even though I have managed to win.) *shakes fist at Castlevania 64, which ends early if you play it on Easy, and the game and manual give you no warning about this*

Except that you can't do Time Trial or No Death Mode with invincibility or slowdown enabled.

(By the way, one criticism of this feature was the description describing the game as being for "disabled" players, to the point where even the author regretted that bit of text; in 2.4, released 10 years after the game's initial release (IIRC), that message was changed to something better.)

Celeste also has a similar assist mode, though there is is clearly separated from options like removing screen shakes. (It appears that the game launched without this option, but someone asked the dev to add this option, and she added it.) Yes, that's a good thing to do, allowing the player to disable elements of the game they don't find fun. I wish more games would do that.

Even the Ocean has a story mode (removes all the action/puzzle sequences from the game) and a gauntlet mode (removes story and cutscenes from the game). I saw someone speedrun gauntlet mode once. (There's also some options that affect the difficulty, like the option to disable death if your health gets too far away from the center.)

Actually i wondered if someone would do an inverse growth. You start at max level and you lower yourself weaker, the weaker you are the more points you get or some paths open up that weren't available before. Maybe starting as a flying god you'd miss a path that a trap-door in a dungeon leading to the real princess which you'd otherwise miss because you'd auto skip most traps.

It does mean the first play-through at full strength you'd be like superman and little has a chance to actually hurt/affect you, going to the other extreme of a level 1 plebeian using rogue tactics and being super careful.

I kind of like the way Field of Glory 2 handles its difficulty setting. No, the enemy units won't have a boost in terms of the damage they deal to the player units, and they also won't become any smarter in higher difficulties as well. What happens when you increase the difficulty of the game is just that the enemy will a bigger disposal of units, therefore you'll be fighting a bigger army as compared to when you're playing on a lower difficulty. That said, the game does alter the quality of the units slightly in both extremes of the difficulty setting (the easiest difficulty and the hardest difficulty), while the difficulties located in between won't do that.

Simulator-ish games (in a terribly broad sense) have the best options, being able to complexify the models. Bullet drops, wing effect, etc for long distance shooters. Stall limits for planes. Fields and speeds of detection for stealth game (including submarine games). Requirements for repairs/healing. Limited information. It's a play on gaming conventions versus realism.

Otherwise, save games limitations is a non-intrusive classic.

Worst solutions are the increase of bullet sponginess. An interesting question is : which difficulty should feel the more real, the more natural. Should easy/hard levels be the ones that feel more artificial, or should the hardest one be the one that feels the most logical ("true" difficulty) ? Probably depends on the genre...

Actually, I am of the opinion that they can be quite intrusive.

If a game does not allow the player to save frequently enough, then that's a game I don't consider to be playable.

If a game puts limits on the number of times one can save or load, or automatically overwrites your save without permission, that's generally a deal-breaker for me.

Edit: I am of the opinion that, if the save system can be adjusted, that setting should be independent of any other difficulty options the game might have.

Hm. I'm pretty sure many 4x games do alter the mapgen, usually one of the last passes, based on difficulty settings. More/fewer resource tiles spawn near the player starting location based on it. I've seen it at least once somewhere. I don't think it was in Civ4 (though Civ4 is coded to pad a player's starting location with a minimum "value" of resources, so if it pops the player on a peninsula with nothing else, it will add A LOT of [low value] fish to make up for it. Oh, Stars in Shadow docs list "Hard Difficulty: The AI factions receive... improved starting positions." The game's all modified-LUA, but it's really spaghetti. Maybe I should try to trace what it does some day.

Still on strategy games: I appreciate it when you can also set the level each opponent separately. So instead of overall "I'm on hard now", you can set one or a couple of them on hard, and others lower. It makes for a fun experience.
---
How's everyone feel about dynamic/adaptive difficulty? It's usually [only?] used to lower the difficulty of bit when the game detects you are failing at a particular sequence a lot. (These games usually give you the option to disable the feature.)

Let's focus on real games with it, not expletives like Candy Crush that use dynamic difficulty to exploit the player to the threshold of frustration-spending. Or EA Sports games using it to fuel loot box spending.

Oh, One Finger Death Punch 2 is entirely dynamic difficulty: Based on your score/success/failure on a level, it will boost/lower the game speed after each stage.

It can lead to rather counter-intuitive styles of play, where the player intentionally plays badly in order to make the rest of the game easier (or faster, if the player is a speedrunner).

I've heard of at least one shmup where, if you play too well early on, the difficulty becomes unmanageable later on.

The problem is actually similar to the issues caused by level scaling in games like Final Fantasy 8 and TES: Oblivion.

Nuclear Blaze (platformer where the player plays a firefighter) has a difficulty slider and several toggles to change specific aspects to tailor the difficulty to the player's preferences.

And it features a kids mode (aimed at about 4–6 year olds) which has other levels, no death, auto-aim (for water spray) and auto-jump/auto-climb, so there are only left, right and spray water buttons left.

Some versions of Dungeon Master, like that Apple 2GS version, offer a kid's dungeon that's like that. Mechanics don't change, but the dungeon is extremely simple, and you're basically handed powerful items you can use to kill the dragon easily. Just be careful not to lose your stuff to the thieves that are quite prominent here.

Oblivion making the game harder if you know how to build a character was one of the most gloriously dumb developer decisions ever. And then they were rewarded with a massive mainstream hit, lol.

Ideally, everything that can effect the difficulty of a game would be in a settings panel somewhere so I can fine tune exactly the experience I want.
Failing that, some form of generic option to make things easier/harder is the absolute minimum.

Being able to turn off intrusive visual effects (screenshake, whiteouts etc) is a 100% necessity if they exist.
No, I do not care in the slightest about anyone's "artistic vision", this is basic accessibility stuff.

Anything that the player must find and use in-game to alter the difficulty/remove effects, is not a difficulty level. It's content.

The "improved starting positions" in the harder difficulties for SiS don't alter the mapgen at all (all but 1 of the races have fixed starting sytems for example).
All the AI boosts are economic - more buildings, more ships, more money, free science, research discounts, ship cost discounts.
Simultaneously the player starts with less stuff as the level increases (on the hardest level you don't start with any industry!), so while the underlying game itself doesn't change with difficulty, the change in the economic balance-of-power means the AI gets a faster start, gets ahead early and becomes much more aggressive because of it.

Depends on a lot of factors. The best would usually be better AI the higher the difficulty is. Alternatively, more/harder enemy types present in the levels or more limited supplies/higher merchant prices. But that all can be very time consuming.

The good old damage/HP scaling is usuallly good enough if not pushed to extreme degrees.

What about NOT preferred methods?

1. Made-up immunities: I believe the 1st game that did this brutally was Diablo 2 and it worked. In Nightmare and Hell difficulties monsters (especially Bosses) would have huge resistances and immunities. Since this was a huge success, many other games tried to implement it. However, they forgot that Diablo didn't actually have any difficulty setting. Nightmare and Hell were practically next levels, the continuation of the same game.

TLTR: I hate it when I increase the difficulty and an enemy is suddenly immune to something that it didn't before.

2. AI cheats with huge starting resources. That mostly happens in 4x games like Civilization or Master of Orion. The opponents start with wealth or technology that normally takes many turns to access.

But I don't think this is similar to having higher HP or attack because strategy games are collective games. There's no escape option when you are trying to defend your city with archers when the enemy has tanks.