Gamerzilla is my open source game achievement system. Originally it was tied to Hubzilla but I recently created a standalone server in .Net. Itch.io could integrate something like that into their site or simply have other sites run achievement servers. We just need game developers and game launchers to support the system. (Technically you don't need game launchers. I just find it easier to have login information in one place instead of adding it to every game.)
My current achievements are available on my test site.