EDIT: Seems dynamic music is back in style in some very recent games, many of which I haven't really played yet. Good. 
For me, it's dynamic music, the kind that some games had that adjusted moment by moment to what was happening in the game.
The best-known example of this in the 90s game TIE Fighter, where the moment more enemy (or allied) ships showed up the music would have a little additional flourish to acknowledge the shift in battle. There were pre-battle tension tracks, battle music, complications of battle, grandiose flourishes for the arrival of enemy or even allied capital ships, and victory and failure music all ready to flow into the next seconds of the game.
A lesser-known but still excellent example of this was in Ultima Underworld and its sequel, where drawing a weapon had its own special "preparing for battle" tension music, getting attacked had a jump-out-of-your-skin joltingly sudden musical start that actually scared me as a kid when I got ambushed, music for battles going well, going poorly, victory and defeat.
I wish more games did those sort of second by second musical changes, but they've sort of fallen out of fashion for the most part. 
Level builders/editors
Splitscreen
K/D ratio - I know it's still in a lot of games but quite a few mainstream shooters have removed it
Aim assist being in everything - if it's crossplay, sure, level the playing field, but if it's console v console there's no need for that shit.
I love aim assist. It just needs to be reframed as an accessibility option. Especially with gamepads, having some tracking and magnetism makes games where you have to aim so much more enjoyable. My wrists and thumbs just aren't doing the same things any more as when I was younger.
It's probably not easy to find the balance, but in any single player or casual multiplayer game it is a very welcome sight.