I'm honestly struggling to think of anything big. I don't know if, fundamentally, much has changed at all other than graphics. Certain styles of games have gotten more refined or gone in and out of style, but I don't know what gameplay we have today that would've seemed absolutely impossible on a PS2, GameCube, Xbox, or circa-2004 PC.
Honestly I think a much more interesting year to go back to would be 1995 or 96. We were just getting comfortable with 3D games back then so a person into gaming in 1995 might be able to interpret what's being shown to them from a 2021 game, but still be amazed by what will eventually be possible.
For 2004 though, I guess No Man's Sky and Elite would seem really impressive and my 2004 self hadn't heard of Elite. Maybe seeing Breath of the Wild specifically running on a handheld would be dope too.
Edit: I guess third person shooters went through a quantum leap in refinement immediately after 2004 (the basic control concepts of modern TPSs were already there in 2004 they just hadn't been as skillfully put together) but that's about all I can think of that would feel really new.