To go a bit further. Are you imagining this is something like I used to see in flash games where some kid would make a pretty decent game but the MS paint art held it back (or could really pop if they got some good stuff), but for the NES and SNES or megadrive?
Say something with a giant out or proportion head, but not in a still looks cool like River City Ransom way, and you want to shave a few pixels or change proportions to get it nice.
Maybe put a nice bit of shading (hoping any palette swaps fall in line or you handle those as well) on something, or flashier energy things, just to juice things a little more..
For animations I have likedhttps://upload.wikimedia.org/wikipedia/en/a/a6/Lemming_animation.gif
Same number of frames, same tile size. One on the right far nicer to look at.
Repeat all that for backgrounds.
The sort of thing you could say load up a tile editor, be told all the stuff is in these locations, and then go to town on your own time?
I am going to say good luck with that, unless you want to take on the Phantasy Star 3 holy war (I really don't like the looks at all, 2 and especially 4 are great though). There were some ugly games (everybody wanted to have a mascot character back in those days, and being the nineties it was all about 'tude or "kids like gross stuff") but they also usually handled like sludge, had less than stellar level design even if you did make things more snappy in the controls front (such things are eminently doable in hacks, however it is not necessarily a 5 minute job, even more so if you want to play with cameras a bit https://docs.google.com/document/d/1iNSQIyNpVGHeak6isbP6AHdHD50gs8MNXF1GCf08efg/pub?embedded=true
), difficulty was all over the place (assuming it was not a collectathon), sounded sometimes OK actually but usually eh. A lot of those otherwise bad games also often had pretty good art too, especially some of the 16 bit stuff.
Or if you prefer there is a reason most such hacks are decensoring, making things look like the arcade, simple colour changes (possibly as part of decensoring), making things look like earlier or later entries in the franchise, or plain old spoof.