What happens if I upload an incomplete app on the App Store?
You can upload the app without worry. 

If your app got rejected,it won't affect the future submissions and you'll keep the app name with yourself. Your app name won't be taken by any other person.

There is a feature in the itunes-connect to set the release date. Even if your app got approved, it won't be released to public.

if there are major tasks to be completed in the application, it is better to create a new application and make the app-status to waiting for uploading of binary. you'll have 6 months time for uploading binary(setup) after creating the app in the itunes-connect.