Overall this looks good to me.

I have one question, wouldn't it be easier to make DiscordInviteGuild and DiscordGuild share code via inheritance?

This would be preferred but I think it would make the naming conventions somewhat confusing, such as having DiscordGuild inherit from DiscordInviteGuild, which doesn't really make sense. If you were to try and rename it to BaseDiscordGuild, that wouldn't really make sense for it to be a property of an invite. Finally, it could be replaced with just a DiscordGuild with a lot of properties being null, but then again that is an issue throughout the library.

I actually did something similar here, however I don't know if this is a good idea to keep implementing.

If it was named DiscordPartialGuild, it would make sense for me.

If it was named DiscordPartialGuild, it would make sense for me.

Indeed, I would say that's a good idea then unless someone says otherwise.