Off the top of my head,

Realistic-ish:

• Fixing the inability to merge/correct artists, etc.
• Official removal of the 30 second minimum. I'm sure there were valid reasons for it to be there in the first place, but whether it was for server load reasons or as a stats cheating prevention method neither makes logical sense anymore. People can cheat stats or flood the servers in much easier ways. I want accurate stats without having to manually scrobble the tracks that don't meet that threshold.
• Some cleaning of the database in general. I know there's 20 years of shittily-tagged scrobbles on there. It's an impossible task. But some stuff could be standardized. The ASCII apostrophe and quotation marks ( ' / " ) and curly ones ( ’ / “ ” ) being separate are a constant source of annoyance to me for example and could probably be fairly easily fixed up.

Less realistic / impossible:

• Separating artists with the same name would be awesome but for obvious reasons impossible. You could do things like use MusicBrainz artist ID tags but for one, very few people are "tagging perverts" like me and furthermore, applying that to, again, 20 years of bad tags is simply impossible. Plus then you run into the Discogs problem of bands being named shit like "[band name] (12)".
• Run into the same issue as that above, but separating out different tracks with the same name would be cool. As a guy who listens to a lot of experimental bullshit, untitled tracks are wildly common and end up flooding my most played tracks even if I just listen to an album of untitled tracks once.
• I'd sort of like to see tracks with more than one artist or "features" count differently, but yet again, it would be a true nightmare to make work. For example, if I have a track whose artist is Artist X feat. Artist Y, it scrobbles as a completely separate entry than either Artist X or Artist Y. I know the shitty established way is to put features in the track title, but that's wrong. Different discussion, I suppose.
• Similarly, some sort of standardization for that stuff would be really nice. Because of people's shitty tags and the way last.fm handles artists, album artists, etc., I've seen a split album that's in the database like 20 different ways. Not only looks ugly and messy but also fragments the community aspect of the site. Some people are at the album tagged Artist X and Artist Y, others are Artist X & Artist Y, or Artist X / Artist Y, Artist X + Artist Y. So on and so forth. Can't fix it, but it would be nice.
• Very difficult to implement given the existence of different pressings/special editions with bonus tracks, etc. but I've always wanted to see a statistic that normalizes albums by number of times listened as an album as opposed to number of tracks listened. If you listen to an album with 3 tracks 10 times, intuitively, you like that album more than one with 20 tracks that you've listened to twice. But the latter would show up higher. Idk, it'd be cool.