A videogame that this description would fit?

Well, it doesn't fit the description of All of your requirements but the Mass Effect series (3 games) is really long and strategic and has a whole bunch of sidequests to keep you occupied for a long time.

Only one I know of that fits all that description is World of Warcraft.
1. It's open world to an extent ( raids and dungeons and pvp ) is NOT open world you que up/group up for them but SOME pvp is open world.

2. It has unlimited playability literally hard to complete every aspect of the game per expansion sadly you need to pay a subscription. Some people only mainly focus on 1 aspect pvp or pve and even then you might fall at the very bottom or can compete for world first guilds and all that stuff it is VERY competitive.

3. Regardless it has gotten toned down over the years trust me you still need to think for yourself and have skills to play. There's a reason why many people don't push for world first raiding achievements and make it into the top 10 yet many people complain this game is easy ( it's nowhere near easy when doing competitive content )

Wildstar is the same as well but both have different play styles. Wildstar seems to be more active while WoW seems to be more strategy based

Elite: Dangerous, except for the "moddable" part.

Unlimited playability, open galaxy, as competitive or cooperative as you want, skill-based combat with knowledge-based tactics.

The only game I can think of that fits all but one (open world): Valve's Team Fortress 2.
It's a team-based tactical game. Technically a first-person shooter, but really, there are only about five weapons that actually shoot bullets. It uses Valve's Source engine, thus has loads of mods.
The fun thing about Team Fortress 2 is that a team in this game is worth so much more than a collection of individuals, more so than in any other game I've played.

Things to keep in mind while playing are:
Should I defend or attack?
What class should I be?
What route is least dangerous?
What team action can overthrow the opponent?
How do I manage my team in such a way this action happens?

The game has a competitive scene in both North America and Europe (to give an idea of the skill ceiling:3). In more competitive games, not necessarily planned and recorded matches, people make the best use out of their headphones in order to communicate threats, possibilities and suggestions with their team.

I've played TF2 for 2382 hours in total and I have no regrets.

Dark Souls/Demon Souls/Bloodborne or Total War if you're into strat games