gamers should own the in game items like clothing, weapons, etc

Sure, though isn't it weird how they never want to allow us to own the copy of the game we bought? Game companies are saying we can own assets but we have a license to the game that they can revoke at any time? Even on physical media??

and be able to trade them with other games.

Yeah interesting idea and eventually doable but unless the games are very similar and are built from the ground up to allow this, it is very difficult.

Even a gun or a sword in one game couldn't directly transfer over to another game even if they are built with the same game engine due to how games need to be balanced.

Think of it in the extreme like dropping the BFG into a wild west scenario ;)

But in my game of D&D, I can't really have a character from another game bring his 20th level sword of vengeance into my game where the players just reached level 5