Alright; right this moment I do not have time to talk on a voice chat service since I'm in an office, but I can give you some general advice.  

Every single language has an ["awesome"](https://github.com/vinta/awesome-python) github repository (Just google "Language name + awesome").   You can use these repos to see the types of libraries are popular for the given language you are working in and use that for ideas.  The one I linked in above is one of the "python awesome" repositories.  You can use this repo to get general ideas on projects.  For instance, if you want to focus on data science, you might want to do something in TensorFlow or Theano and if you decide to do a web app, you can also choose to build a project with a web framework like Django or Flask etc.  

Its also a good idea to go through github repositories and look at various programs that exist in the target language.  For instance, with rust, I might look for implementations of a specific command line tool or a game similar to snake and I will then take a few notes from the code base.  Often times, you will find fairly large programs so its up to you to scale them down for viewers. 

The rest of the stuff that I choose, I just choose through experience.  I've had real world experience with all of the languages that I teach.  That makes it easier for me to know what kind of programs I should and shouldn't try to build.  

For Java, C# and Python, you have many similar choices.  They all have many great UI frameworks and you can use them to build android applications as well.  You can do Web apps, games and data science in all of them.  They are all well established programming languages with very good documentation.  I would also recommend you look at other tutorials and use them to get a little bit of inspiration.

If you have any questions feel free to ask.