I've designed to all sort of models. Sometimes you build "personas", and try to view the requirements for each, and choose the key features for each of them.
Other times, if it's a really specific/targeted app, the "what would I want this to do" works well, particularly if it's an area where you see a "gap" currently.
For me, in a battery app, I want to be able to view it on the lock screen and home screen, and have it be current to a given time-slice (i.e. 30 seconds, 1,5,10,15,60.... minutes, or whatever). Clearly, frequent polling would be a battery problem in general.
Currently, I don't know of any battery apps that function well on W10m. I finally un-installed the two I had used for several years, as they weren't really much more accurate than a random-number, with about a 25% variance of the actual battery.
Simply having one that was accurate and viewable would be my primary feature.
From there, being able to track and isolate/graph high-drain issues would be interesting, again, if it were "accurate".