What is it that makes a RPG and RPG, that is what I want to understand better.
What are the basic foundations that determine what is a RPG?
And how has the original RPG evolved into nowadays RPG?

How does a RPG differ from a JRPG?

What can be done to elevate the RPG (or JRPG) experience?

What has been done to make the RPG (or JRPG) experience worse?

What games have made RPGs (or JRPGs) better? or worse?

I hope to learn more and expand my perspective.

I'd argue for an originalist interpretation that any game whose main mechanics ultimately derive from Dungeons & Dragons is an RPG, and those who don't aren't. Simple.

Of course, these days, a lot of games mix and match genres, so... not as simple.

Couldn’t have said it better myself. The term RPG was been very watered down in recent times, but, I understand those who still refer to a game with “RPG mechanics” as an RPG. 

It’s probably one of the broadest genres in all of gaming now though. 

So Dungeons & Dragons is the original RPG. What are the core mechanics of that game?

Personally, I think the main core mechanics that are the foundation to D&D the following:

• Exploration
• Social Interaction
• Combat

Am I missing anything?

What have RPG's done to expand on these mechanics and anything outside these?

It's hard to say, because the meaning has kind of changed throughout the years.

The term comes from pen and paper roleplaying games like Dungeons and Dragons. So, the first roleplaying games were games that used a lot of those elements. I.e. a turn based battle system, equipment that alters stats, customizable characters, an element of chance (i.e. dice rolls or a random number generator to determine a hit or miss), classical fantasy setting, number based stat systems, an inventory system, skill trees, leveling up etc. etc.

In video games, the RPG term tended to be used to describe games with similar elements to games like Final Fantasy or Dragon Quest. For example, elemental based magic systems, fantasy setting, a focus on story, lots of text/cutscenes, and so on so forth. Especially in the 90s when lots of games didn't have extensive stories, this came to be associated with RPGs. Also, games that were more open, were often considered RPGs. So, for instance, Ocarina of Time is sometimes thought of as an RPG because it has some similarities with things like Final Fantasy. The setting, elemental based magic, and a focus on story, and a relatively open world. This is despite the fact that in terms of the way the game plays it doesn't have much in common with classic pen and paper roleplaying games.

As time went on, the lines between the two began to blur. RPGs began to be less turn based and more and more action oriented. So, something like Kingdom Hearts, which maintains some elements of an RPG (elemental magic, MP, leveling up, equippable items/skills, summoning spells), but really is far closer to an action game in many respects. That is considered an action RPG. On the other side, you have games like Spider-Man or Shadow of Mordor which incorporate RPG elements (skill trees, stats, leveling up) but are generally not considered RPGs.

A JRPG generally refers to games that specifically build on the foundation of Dragon Quest and Final Fantasy. So, more often party based, with a set path, magic system, summons, etc. Especially if they are turn based, but that's not really a requirement. And, of course, it helps if it was made in Japan. They also tend to be longer, with elaborate stories, a heavy focus on cut scenes, often with heavy influence from anime (i.e. Xenoblade). On the other hand WRPGs tend to take more inspiration from high fantasy (think Lord of the Rings), have a less linear story, usually focus on one customizable main character, and so on.

That's a roundabout way to say that there's really no great answer to the question. It's kind of like a game of telephone, or maybe like evolution. Originally, it denoted something that was kind of like a video game version of table top RPGs (and this is still true in some cases like Baldur's gate). But, then as time went on, the traits of early RPGs like Dragon Quest or Final Fantasy became incorporated into the video game definition of RPGs. So, then games that have those elements that were added to the definition are considered RPGs, even though they don't have much in common with table top role playing games. Eventually you get to a point where something like Mass Effect 3 is often considered a roleplaying game, although what makes it a roleplaying game is questionable at best.

One of the biggest characteristics I’ve always preferred in my RPGs are choice/consequence.

It’s a feature that unfortunately tends to be missing in most RPGs these days. If I wanna be the most evil person alive, let me! lol

Just like how "Shooters" and "Action" games are a lot more broader in what types of sub-genres are available, RPGs are like that.

Mainly its jRPGs (Japanese RPGs) and wRPGs (Western RPGs).

jRPGs are more focused on a single plot line that usually consists of a group of characters fighting against an antagonist that threatens to destroy the world with the inclusion of some minor side quests that have little to no impact to the main story. Typically turn based but also expands to action or MMO.

wRPGs are more focused on the a individual character (usually custom made) that follows a main story that is usually put on the back burner in favor of side quests that have their own plot. Typically action based but also includes different types of PoV/further sub genre such as table top style, isometric rpg, etc

At least that is what it was originally, now the two sub-genres share similar features - now its just a term used for where it was made but some indie groups create a game in the old school definition terms to cater to a minority group of players.

Levels. Stats. Number-based health and damage (HP). Character creation and customization. Narrative-driven with relevant player choice and/or input, that one being more difficult to implement in video games due to budget and time reasons (on the other hand, HP exists in virtually any game even as a hidden mechanic due to that just being how software works).

About 10 years ago I tried to come up with a definitive definition. It’s not perfect, but I think it’s a good jumping-off point:

An RPG is a game that features 1) a set of tangible, appreciable statistics that directly correspond to intangible qualities like strength, charisma, and wisdom, 2) the ability to modify those statistics by earning experience points by defeating enemies and completing quests, and 3) the electronic simulation of die-rolling mechanics from pen-and-paper RPGs like Dungeons & Dragons to determine success and failure.

Original D&D was vastly different game than what it is today.

It was basically a game of survival horror - you enter the dungeon, where most things want you dead, with limited resources, and you're trying to get the treasure, using your wits and with as little combat as possible. Cause that dungeon is usually very dark, death is around every corner, life is very cheap, and you don't get any XP for killing anything. Only XP you get is gold you bring back to town - literally, 1GP is 1XP.

Other thing that was different is the way it was played - OD&D was played more like MMO, in clubs, you play in some world made by DM and there are many parties playing in the same world (either helping each other, or not). There is no predefined story, you play the world, stories come from interacting with the world and world interacting with characters. Actions trigger reactions. Choices and Consequences. Game is open ended, with PCs eventually retiring (if they live that long) and becoming (in)famous NPCs in that world, that future PCs might (or might not) interact with. "There must be a story" D&D era came much later with "Hickman manifesto" and Castle Ravenloft, which many consider turning point for D&D (for good or bad - I'm in latter camp).

In OD&D, role playing does not mean what it means today - D&D was created by wargamers, from wargaming rules (Chainmail), so you play a role in your squad (AKA party). That's the "Role" part. Later interpretations came...later.

Abilities are fixed once you make your character (Ability Score Increase came only with 3rd edition of D&D and onward), your character evolves through items and level based class improvements, and, beside Thief class, there are no skills. Focus is mostly on player skills, instead of character skills.

Later in 70s, another game came out that helped define RPGs - Chaosium's RuneQuest. In RuneQuest there are no classes and levels, and whole game is based on skills that your character has (a subset of so many that exist in the game). Unlike D&D, where combat is based on your ability to overcome enemies AC (Armor Class) and score a hit, in RuneQuest combat is opposed skill roll between two combatants and armor has a role of damage reduction. Progress is made by increasing your skills and focus is more on your character skills, instead of player skills. Game is open world, as well as open ended.

I'd say both have influenced VG RPGs quite a bit, though mechanically I'd say RQ has one up over D&D in VG RPGs in the long run. D&D is of course still most popular TTRPG, but RuneQuest mechanics spawned Chaosium's Call of Cthulhu, which is most popular non-D&D derived TTRPG.

As for VG RPGs, unfortunately, there aren't many VG RPGs that pass the "RPG test" of creating you character, having that character grow via players decisions and having that characters choices have consequences in gameplay world, while having freedom to do whatever you want. That is, however, somewhat understandable, given that VG RPGs can never reach level of decisions and interactions that TTRPG table can have with Game Master running the world, thus always lacking key RPG component, in addition to being constrained with budgets it takes to make such games. But eventually, with advances in AI, we might get equivalent of GM in VG RPGs and proper TTRPG alike experiences.