The rules for a general RPG should have no lore in them. Most RPG lore is just something that some people made up, and in order to really get into it you first have to learn the lore that some rando thought up.

But RPGs are all about the players creating a story. They, in concert with the game master, should be the ones coming up with the lore.

This doesn't mean that there can't be any lore, just that it shouldn't be part of the game's rules. In fact, having separate lore/world documents available for those who want or need it is fine. There might even be a market for fans to contribute their own.

I am coming to believe that most people conceive of D&D campaigns that are too long. Characters advancing through twenty levels and campaigns lasting years can make for great videos, but most people don't have the time for that. And all the abilities and spells that characters can have at the higher levels is overwhelming for most people.

So I am thinking that character advancement to maximum abilities should take around 6 months of active regular play. That is something people are more likely to be able to handle.

I am also thinking that the amount of abilities should be much more manageable. In my current D&D classes I have been printing out cards for each of a character's abilities, and for some characters they have several dozen cards. That is way too much to choose from during gameplay, so another goal is to make the ability/skill system such that even if they have a lot of choices when creating and growing their character, they never have more than about a dozen abilities.

Using a skill should be required for getting better at it. Conversely, if you don't use something you shouldn't be getting better at it. In practice this means that something like a proficiency bonus should be tied to each individual feat/skill, and should only be increased when that has been used a significant amount.

It doesn't make sense that a rogue that never uses their Sleight of Hand skill, for example, would ever get better at it. In fact, it might make sense that something they have a proficiency bonus in but they never use, might get that bonus reduced - but I would limit that to maybe one point and only over a long period of time.

Perhaps the number of times you use a skill required to advance proficiency is tracked exponentially.