I just make what feels fun. For a perspective on development cycle: the initial version of Agar.io took 1 week, while Diep.io took less than a month too (it was released a lot more polished than agar.io). Of course, they continued receiving updates for months, but the initial version was already fun.
While the initial version for both games was coded fairly quick, there were months of thinking about them before I even started coding, and from beginning of development to first version the gameplay plans didn't change.