Which / what applications are you missing on Linux?

I have the whole month off in January 2021 and I want to start a new project there.
What are your ideas for me? or which / what kind of applications you would like to have / use on linux that doesn't exist on linux or there is only a small selection for it outthere.
The project shouldn't be very big because I only have a month. If after a month I get good results I will continue to work on this project and publish it as an open source project.