What online course should I teach?

I intend to make an online course but I am not exactly able to gauge the demand. I have worked as both mobile and web developer with around 3+ years experience in both technologies. I am also pursuing my PhD in CS inclined more towards NLP and XAI. I also consider it as my area of expertise. The purpose to make this course to gain experience in online teaching and teaching in general but also to earn some passive income. I have also published a course on Udemy for Laravel 3 weeks ago but it hasn't gained much traction.