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. 
For me the biggest thing is sprite art. God damn do I hate playing 2D Mario with 3D graphics - I've hated it since New Super Mario Brothers, something about it just makes the game look mushy and shitty and generic. One of the best things about Super Mario Wonder is that they put an awful lot of effort into aping the old 2D style with that game's animations and environments - in the same way that for example Arc System Works games ape anime art with their 3D models - and I have to ask why they don't just do high quality sprites from the jump?
Quality sprite work is its own medium in my opinion. It's not longer just "nostalgia" any more than painting with paint instead of an electronic tablet is "nostalgia."
I also like sprite art; it's literal art frame to frame. The original 90's doom is more fun to me than the new ones and in my honest opinion feels better to look at.
This is why unfortunately. KoF XIII tried that and it looks absolutely fantastic, but the amount of time it takes an artist or even a team of artists to do 500+ drawings per character is intense.
High quality sprites are expensive and difficult to work with. Think about if they were to release a new fighter like slayer. The number of hours to get good sprites for thr transformation makes it prohibitive. Also, there are way fewer artists skilled in that anymore. 3d is still cheaper and easier. And, now that they can do it well all kinda of cool options are opened up with rotation and transformation that couldn't be imagined before
Bespoke pixel art is an unbelievable amount of work. I always like the example of Dead Cells where the art guy had too much work to do by hand so he went with low-poly models run through a pixelization filter and exported to a prerendered sprite sheet. Much, much faster than working by hand, and it also allowed him to do things like easily adjust animation timing and minor movements without needing a whole new sprite or a weird visible pause in the sprite animation. It's like the opposite of Donkey Kong Country, which did prerendered sprites for the high amount of detail at the time, he did it because it's so much less work these days.