Here's a collection of interdependent ideas. It's not possible to add this whole thing in the wishlist, so i'm placing it here, hopping someone from GOG will see it and evaluate it.

Settings
1. Be able to define a default "Base Download Folder" for offline/backup downloads. Each offline download would be automatically placed in "BaseDownloadFolder\GameName". This already exists today, however the folder names are named "game_name". Personally, i would prefer "Game Name".
2. Be able to define a default "Base Installation Folder" for game installations. Each game would be installed in "BaseInstallationFolder\GameName". This already exists today.
3. Option (checkbox) to "delete older versions of downloaded setups". This means that if a "setup_GameName_version2.exe" is successfully downloaded, if "setup_GameName_version1.exe" exists in the "BaseDownloadFolder\GameName", it should be deleted. The same should happen for Extras and maybe Patches.
4. Option (checkbox) to "keep 1 previous downloaded setup". This means that if a "setup_GameName_version3.exe" is successfully downloaded, if "setup_GameName_version1.exe" and "setup_GameName_version2.exe" exist in the "BaseDownloadFolder\GameName", version1 should be deleted but version2 should be kept. Optionally, the user could be asked after i.e. 2 weeks of playing the game, if he wishes to keep the previous version of the setup or delete it. This would be helpful in case a release is problematic and the previous version is not.
5. An option which when a game is installed, an uninstall for it to be placed (if checked) or NOT be placed (if not checked) in Programs (Control Panel). Sometimes, when having many games installed in a WORK PC (because you don't have any other choice), it's not the best idea to have the Programs list cluttered with them. For each game, the user would still be able to uninstall through Galaxy, as if it were in Programs list and he pressed the "Uninstall" button.
6. An option to "Automatically download backup of game on Install". Analysed in "Installing.1".

If (3) and (4) seem too much, at least an "Open Backup Game Folder" button should exist (like the "Show Folder" that exists now).

Backups & goodies
1. Each downloaded main game installation should be placed in "BaseDownloadFolder\GameName". The Setup filename should be something like "Setup_GameName_version.exe".
2. Each downloaded patch should be placed in "BaseDownloadFolder\GameName\Patches". The Setup filename should be something like "Patch_GameName_version.exe".
3. Each downloaded extras file should be placed in "BaseDownloadFolder\GameName\Extras". The filename should be descriptive of what the Extra is.

All of the above should preferably be in a compressed format (zip or even better, 7z). It would be ideal if the user could choose the format from a short list of supported formats (zip, 7z are the most popular nowadays).

Downloading
1. Ability to pause a download / reorder the download list.

Installing
1. Galaxy should first check if the "Settings.6" option is checked. If yes, when the user presses the "Install" button, the main installation of the file should be downloaded (and saved in "BaseDownloadFolder\GameName") and when finished, the installed should automatically be launched (as happens today). Optionally, the user could be asked if he also wishes to download and save in "BaseDownloadFolder\GameName" any extras before the actual download starts.

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I think a "Nuclear Bunker" option would be interesting, where you click the button and Galaxy downloads a backup of every single game and bonus item in your account. Like, if I just decided, "I wanna throw everything I have on this SSD," it'd be cool if I could do that without any fuss.

If I was making it I would make sure it's offline-first. Meaning it would start up nearly instantly without waiting to connect to an account on GOG. There would be a local profile that would keep track of every thing you'd want to keep track of, which could then be linked to a GOG account when you can connect to the server. The syncing of the client with your GOG account can happen in the background after it has finished loading if you're on the Internet, but I'd have it perfectly functional even if you never connect it to the Internet at all ever.

I'd let it handle matchmaking and host multiplayer servers over a LAN. Multiplayer without Internet is one of the big reasons getting games from GOG in the first place, so I hope we can pressure GOG (or reverse-engineer something) to make this one a reality.

Keyboard accessibility.

I'd love if Galaxy had an option NOT to announce to friends when I start playing a game. I like Galaxy, but that seriously freaks me out, and I tend to avoid playing with Galaxy because of that.

Got a whole lot of ideas for handling of updates, but that's an essay on its own.