To some extent, I don't think we could make a standard set of features apply to all games or game genres, even though a lot of those games blur the genre boundaries. Some things you folks touched on:
1) Ability to easily respec character--this lets you have fun experimenting with different playstyles.
Yes for the most part, at least in how it applies to RPGs & ARPGs. It can be pretty easy to create a character, or group of characters that just can't hack it, or could be better, and respecing mid game would be nice, although I think there should be a cost or penalty to doing it more than once. As far as doing different playstyles, many games allow you to build different characters and have saves dedicated to them exclusively, like in the ARPGs
Diablo 2 Resurrected or
Grim Dawn.
2) Multiple save slots.
I'd also like to add, for the love of all that is holy, the ability to manually save your game (there was a thread somewhere about this). Maybe this one isn't for absolutely every game,
Manual saving. We all know why checkpoints are an industry standard, and it's because there aren't enough buttons on a controller for a quick-save key.
Multiple save slots, quick saves, and manual saving are huge pluses for me in any game, and away from that whole check-point save mentality that many "AAA" games use now. I understand not being able to save at certain points (like combat or cut scenes), but if I need to quit the game at some point for whatever reason, I want to be able to return to that same point when I go back to the game. "Save on Exit", which saves the game state, works in some ARPGs like
Diablo 2 Resurrected, but it's rare that I'll play a game for long that doesn't have manual saving. How many times have you played a game late at night wondering if you could hit that check point save before you really (really) needed to go to bed?
Better Difficulty settings, probably best set in an options screen. I usually want more enemies, rather than making them stronger or me weaker—or making my upgrades more expensive.
Multiple difficulty options are big with me, and I think it's important when we take into consideration accessibility for as many people as possible. It might not work for all games, but from an RPG standpoint I always appreciate those different options. It's finding that comfort level that makes a game fun, yet challenging, for each of us. I have no shame in saying that in my first
Witcher 3 playthrough I played on "easy" and loved it, but was still challenged. Also, I have to agree that facing mobs of enemies is preferable to facing a bit bullet sponge boss.
Procedurally generated maps—nothing like having every playthru be different.
No, I don't think that works in most games, whether open world or more linear. It worked in Diablo 2, but that's a rare exception. I think that Bioware was trying to do that in
Mass Effect Andromeda, but gave up on it (they had other issues anyway).
Mod-friendliness! Design games so bits can be extracted and replaced by a modder's offering, and the installation can be easy for non-techie players—eg via Steam Workshop or similar.
Mods already replace vanilla in every game I've ever played, but making each game more mod friendly would be fantastic. There needs to be a system of game development that doesn't break mods with any decent update. That's not really an issue with games like FC, but a lot of other games break the mods over and over, and you end up losing a lot of great mods through the years because the modders stop fixing them.
Making a game mod friendly is great, but it is NOT easy! Particularly for tech support. There are many people out there who "just know their mods aren't the problem" and so will lie and say they don't have any. It's no picnic for coding, either. Asking for it to be in all games is asking for all games to have fewer features.
The number one problem here in terms of modding is the game engine (I think that's the correct term) varies greatly from game to game.
Skyrim uses the Creation Engine which is based on the Gamebryo Engine,
Dragon Age Inquistion uses the Frostbite Engine, and there are many other variations or types of "engines" out there, that making them all easily modded would be impossible. It would be awesome, but I don't see it happening.
Another thing you'll never see, is a game developer offering tech support for a modded game. How could they? With all the mods and mod combinations it's just not possible. That goes hand in hand in a game update "breaking" a modded game. A game developer can't patch or update a game while considering every gamer's mod list. If your game is on Steam, there's really no excuse for your modded game to be "broken", as you can set it to "do not update".
Modding is one of the best benefits for playing games on a PC, but there will never be a unified standard for mods or modding.
NOTE: Sorry for the long post, I've been away for a few days. Nothing new to add at the moment, but felt the need to comment.