There are many of open source projects around the world and people usually contribute to what they love or use at work. Because of that it is hard to answer what are good projects exactly for you.
You can take a look on my tiny vanilla library https://bunnyjs.com - the idea is described on first screen and it says everything you need to know. I am always opened for any suggestions, ideas, feedback but real help of course is unreplacable.
You can also add me on Twitter or Facebook /mevrael and I will tell you some very interesting other vanilla JS projects.