Which stages of the requirements analysis process in mobile requirements engineering are the most challenging ones? [closed]

Question

I'm doing a research on formulating a requirements analysis model as a stage of requirements engineering for mobile-application development by considering the limitations and the needs of it ( agility and etc.. .), what I'm trying to figure out is that which parts of this process (requirements analysis for mobile development) are the most challenging ones ( so i can focus more on) , and if there is any stage that u think I need to include or exclude (exp. some may think a quality plan may or may not be necessary and etc.)

to make it more clear below is the list of few of the areas in which I can focus on ( by the way your suggestions can be anything out of the below list.)

-Requirements specification -Prototyping -Requirements Prioritization -Focusing on quality functions