Ask HN: Part time project ideas

For some personal medical reason I won't be able to join any job for next 4 months. But I would be able to manage to spend 4 hours a day for something I love which is building usable software. Can someone give me an idea ? I am not expecting any sort of income out of this. It would be good if I spent this time building something from scratch which can be usable and cannbe open sourced to community. I have around 11 years of experience in IT industry. Thanks in advance.