How do you know an idea to write about is “good”? What is the hardest aspect of making an idea a complete paper?

I know an idea to write about is "good" if I have lots of things to write about. Also, if there are many datas, it becomes better. In addition, if there are many opinions about it, it is good chance for me to persuade others to come to my side. Hardest aspect of making an idea a complete paper is getting shocking but also reliable datas. Also, making my piece all coherent difficult for me because I usually get out of topic that I'm writing about.