The hardest part of making a mobile app for your web service is feature parity. For any large scale web offering, it’s near impossible. Talk to your users, observe them in their work environment, and prioritize the features that are uniquely valuable on mobile.

For the rest, make sure your web app isn’t broken on mobile web and punt to that. 

 Having an API for third party devs is great too as it allows small groups of people to get their pet feature built out on their own.