I've recently moved to Lunar Client from Vanilla Minecraft with mods, and honestly it's indescribable how much better Lunar Client is. However, there are some things that I would like to see in Lunar client. To preface, these changes can be made by installing fabric mods, but I would prefer if they were just part of the client to begin with.
Animation Management: Similar to what sodium has, allows us to control what block animations are enabled
Particle Management: Similar to what sodium has, allow us to control what particles are displayed, make them be shown more or less, currently only able to increase amount of particles
Sound Management: Edit to current Better Sound module, all sounds should start at 50%, so we can turn up some sounds without having to turn every other sound down to make it louder
Armor Hud: Edit to current armor hud module, make multiple presets, some like uku's armor hud, some like inventory hud+'s armor hud, some like simple armor hud
Zoom: Edit to current zoom module, add an option to make it not effect hand fov, also change zoom in and zoom out speed separately
GUI Scaling: Allow different aspects of GUI to be edited individually, inventory scale, hotbar scale, menu scale, etc
Shulker Preview: Similar to Simple Shulker Preview, add an option to add a small version of the first item in the shulker to the icon and shulker capacity bar
Shift-Click Rebind: While in inventory being able to rebind shift-click to a key
General Optimization: Take some ideas from current mods, like optimizing server routing, reducing effects of ping (clienthings), tick syncing client and server, etc
3rd Person Eating Animation: pretty self explanatory, would apply to every player, not just yourself
Crosshair: Edit to current crosshair module, similar to walksys crosshair addons, when an entity comes into your crosshair and is in hit range, the crosshair changes (color, additional lines or smth, etc)
I'm not sure how difficult implementing these sort of features would be, but it would genuinely be amazing to see it happen. This client is awesome already, but these few changes would make it perfect (at least for me).