Sure, lots of people have suggested this already, along with using 2d10 for a non-linear distribution with a slightly wider range.
And it is a fix for the "randomness" of D&D, where the roll of the d20 overshadows most abilities until like level 10 to 12 (without expertise) or maybe level 5 with expertise. But also, for a lot of players, the d20 is more fun, because it is more random. You don't have to build an optimal character to succeed, it is more up to the dice. The Characters are more heroic, because there is the possibility that they could succeed at everything, even if the chance is only 5%.
Both, 3d6/2d10 and 1d20 have its merits and disadvantages.
For myself, even capping at 15 and allowing racial modifiers to bump it to 17 keeps the bonus at +3, so no +4 or +5 to start. Also, in my games when you get an ASI, you cannot apply +2 to one ability, just a +1. So, if you have a 16, you require 4 ASIs to bump it up to 20, which is at 16th level in most cases.
That is fine, too. Your initial starting point is higher for the ability scores, but the rate of improvement is slower, so proficiency catches up and overtakes Abilities by like level 5.
Yeah, as soon as you claim "Lets take a look at the most succesful RPG Game out there: Pokemon" it is really difficult for me to take you seriously... I don't see how "location" should really have anything to do with it other than in-game reasons.
Location is a stand-in for Fixed DCs/CRs for everything in the game world. When I developed that system for the Games I run, it was more a way for the encounters:
In this area of the world, the encounters are more for level 1 characters. If they go to this area, they are more likley to have encounters that are hard for level 4 characters and so on. There can always be exceptions, of course. But in general, this way, I populate my world with areas that are safe or unsafe for certain character levels (and NPCs), the same way Pokemon does. And I know, a lot of people look down on pokemon, but from a game design perspective, they are doing a lot of thins right especially in the way game progression feels. Pokemon Games give you a sense of progression that just works (at least the iterations I played) and that give the players of the game a sense that they have a control over the progression.
This also applies to other challenges. The DC for picking the rusty lock of the old Barn will always be 5 (unless ingame reasons replace the rust lock with a new fancy lock or something), for NPCs, for Player-Characters ect.pp. - the DC for picking the lock of the unbreakable vault will always be 30. So a location in this sense could be the old abandoned farm, where the Encounters are for Level 1, but also the DCs for Checks are generally low (there can always be exceptions), while the other location is the unbreakable Bank, with their unbreakable vault, where a fight would be balanced for a Level 12 party and picking that locks would have high DCs.
A location based difficulty setting can be applied on the macro level (this region is for low level adventures and this region is for higher level adventures) but also on the micro level (don't break into the unbreakable Bank at Level 1 and you will be bored at the abandoned farm at level 12).
Anyway, sure, DC is always the same for a particular task, regardless of the PC level or ability. A DC 20 lock is very hard for a 1st level PC, but becomes easy for a 20th level PC.
Yes, for a specific task the DC should be the same. To pick the lock at the unbreakable vault you need to be a DC 30, to pick the lock at the barn you need to beat DC 5. Thats what you need for a world that feels realistic but also a world in where the characters can grow. This is in contrast to a DM who adjusts the DCs to the player abilities ( my exaggerated example). The Adjuster DM will increase the DC of the Barn Lock to 20, because the party is level 10. He will decrease the DC of the Unbreakable Vault-Lock to 10, because the party is level 1. That's the opposite extreme of my "location based difficulty setting".
If I am understanding you on this, it is already how most games are run. You gain levels, adventures get harder. However, again, location doesn't really come into consideration other than in-game reasons...
No ... the opposite. You reach the minimum level requirement for a region to be able to survive there, you go there (Players can of course choose to go there before there are strong enough, they just then die). The longer they survive in that region, the stronger they become, the easier that region becomes. When it gets too easy, they can go to a new region with harder challenges and it starts anew.
This is of course a simplification of the game process.
Ingame it looks more like this:
"Okay, we destroyed the Orc-Bandit-Camp in the Easywood-Forest, the pitiful rest of orcs that roam this region are no danger to the towns or armed travelers anymore. I think we learned enough and are now strong enough to look at that Undead-Problem in the Hardwood-Forest."
"Couldn't we go the Mountains of Certain Doom to collect the Reward for the ancient Dragon?"
"Only if you want to die ..."
And of course, that are all in-game-reasons. All DCs and Challenge ratings should have ingame-reasons. But when you create your world, your setting, your adventure for the players, you are creating the ingame-reasons.
And the normal way to create an adventure is, if you follow the DMG:
Oh, my level is level 3, so lets put them against an encounter that is a medium encounter for a level 3 party.
Oh, my Players have this und that skills, so I should set the DCs accordingly.
My way is:
This area/location is survivable for Level 1 characters, that for level 3 characters and so on, so I can plan in advance the progress of the game. Its like when you build a megadungeon with 20 Levels and every level is set to challenge a party of the same level. And if the party gains quicker levels, the lower Dungeonlevels get easier and become trivial after a while.
You just build the world as a megadungeon.
Trivial is just a synonym for Very Easy.
Again, you're focused on PCs. The in-game world has lots of other creatures who will attempt to do things other than PCs.
Anyway, you have PCs roll only when there is a significant cost for failure and success is in doubt (they have a chance of succeeding or failing). So, yeah, I know how to read the chart. I just don't agree with the labels corresponding to the DCs and the probabilities they represent.
Then change the label.
The funny thing is, if you assume an average of +5 (which you are doing), you're just shifting the success to what I think they should be at +0.
But only for skilled characters. Skilled characters don't fail at very easy tasks. Only unskilled characters do. Like the guy who can't cook and burned the breakfast ceral.