Open source language learning only has Anki. Everything else is in an enbryonic stage.

There are so many low hanging fruits. Add-on to look up words in subtitles and add it to Anki. Luo dingo clone that's a bit less tedious (without having to write so much of your native language). Clozemaster clone (unless someone knows how to set up Anki to do this)

Google earth

Proper 3D CAD software, we have FreeCAD but it isn't very good.

Another good email client. Many are trying to leave Thunderbird on GNU Linux but there aren't many to choose from.

This is niche, but I really want a good FOSS screenwriting software that can rival Highland. There are some options like Trelby and others (because the Fountain syntax makes interchangeable screenplay files possible) but right now none of them are as good as Highland. A good alternative could let me finally leave Apple