I will put them in order and say some improvement:
-Steam: the gold standard. I would change the following:
- Rework the music player
- Rework community making it more seamless with the rest of the store.
- Add more social features to make sharing pictures and videos easier.
- Take steam.tv out of beta and heavily promote it.
- Better integration of the different sections
- Update the Steam App so that it is not fire trash.
- Improve Big Picture Mode.
- Add native Cheat Engine (or something like that support)
- Improve Workshop so that you can tie mods to game releases.
- Make getting older versions of the game easier (right now you can use the console command to do it and devs can do it through the betas part in the settings.
- Allow more easy customization of the UI (with colors at least easily).
-Gog Galaxy 2.0 - i quite like the new library, but it still feels like it needs heavy stability improvements.
-Origin - its decent, hasnt really improved much since launch. I would change the whole UI and make it
-uPlay - itsOk.jpg , I like their ubi points stuff, even if recently they have neutered a ton of their value.
-EGS - somehow slower than Steam. The new Library UI is OK, adds 0 extra value when compared to uPlay or Origin. Somehow you cant speak with your friends while in different games?
-Battle.Net: Still feels like something that was supposed to be for a small number of games that gets bloater and bloater with each release. It requires some heavy modifications to make it a proper thing.
-Bethesda: just throw it to the bin and do a different one. It is trash.
-Microsoft Store / Xbox App: it is much more stable than it was before. Still the whole formatting of the files makes it a mess and not something I would actively use.
The main problem of the improvements is that I would make most of them add all the stuff from Steam lol