@thomasfuchs another annoyance is PNG uploads containing sharp details getting annihilated by the downscaling and often getting converted to lossy for no reason. really annoying for posting circuit diagrams!
I'd also like to see 10-bit and HDR image support, but this is one of those features that needs to be done right (the people who want this are very exacting!) and I doubt there are many people on the mastodon dev team with a sufficiently strong colorimetry background to implement it well.