For HR, if you could ask every employee at your organization a question or two each week, what would it be?

"What recent work activities have been most fulfilling for you, what has gotten you excited or "lit up"?

This question leads into the territory of Culture and Mission. It'll underpin the dialogue to evolve an explicit Mission from the bottom up, and will also serve as a filter for employees to notice whether what they are passionate about fits with the group mission or passion.

"How can we help you?"

This is broad enough to cover work-related enquiries ("We need more training in X") and personal ("I'm feeling very stressed lately"). Both are important for HR to know and develop solutions for.

To achieve meaningful results however, this question has to be asked of the employees by the managers and leaders of the organization as well, on an ongoing basis. No technology can or should ever replace that.

Although this seems like a good idea perhaps for huge companies, i believe that replacing human contact with an app, would not do much to promote employee engagement. as an employee i would value personal interaction with HR and my superiors to feel they really care about my well being and retention. A company interface might have the opposite affect.

Now if you are talking about asking non business related questions, like "what's your favorite coffee flavor?" (if next to the coffee ;) )or "what kind of company event would you like best?" - this gives the employee a friendlier atmosphere and engages them as part of team. make it fun and morale building as well as informative for you.
hope this helps!
Duncan