I think, in the fields of urban planning and design most of the new ideas today depends on the predictions analysis by using innovative technological methods. The new articles should focus on one or more of the digital new techniques, such as Big Data Analytics, Internet of Things, and Crowdsourcing Approach. Accordingly, try to search for the ideas which are based on this orientation.
Just have to think all the time. When you think and understand, questions arise. The most important thing is exactly formulate this as problem. Then find solutions ways (albeit partial). And then - it's simple - to make and write.
I can't talk about anyone but myself. I do it naturally. Now, for example, I would like to write about 10 articles (there are certain ideas). But for 7 of them I do not have time, partners and equipment. There are 3 remaining problems are possible to solve. I try to implement them, but I don’t know which I will finish earlier.
You could find new ideas in lectures, in conferences or in articles that are in your area of interest. When you have an idea, you formulate it into a research question, that is, something that you want to prove. Then you start writing down the details of your idea, and look for previous papers on the subject, and observe what they have to say on this subject. Then you could decide if your work will be theoretical, or experimental. From this point on, you could see the example of other papers on your subject, and how these papers were written and organized for publication.