I am planning on making a hybrid app. There will be a web mode in which I will show a SAS responsive app and advanced mode that will take advantage of the native capabilities of the phone. The purpose of this is not to rewrite the web app natively on iOS but reuse the existing web app. The advanced mode is simply for features only available via a mobile device. Will an app like this get rejected?
As per the App Store Review Guidelines, yes, this kind of app will be rejected.
February 2016 Guidelines, section 2.12:
2.12 Apps that are not very useful, are simply web sites bundled as apps, or do not provide any lasting entertainment value may be rejected
June 2016 Guidelines, section 4.2:
4.2 Minimum Functionality
Your app should include features, content, and UI that elevate it beyond a repackaged website. If your app is not particularly useful, unique, or “app-like,” it doesn’t belong on the App Store. If your App doesn't provide some sort of lasting entertainment value, or is just plain creepy, it may not be accepted.
However you can reuse your codes by creating cross-platform based apps using some SDK such as: Cordova , Titanium - Appcelerator based on your need and requirement you can go through their documents and choose one.
More details on choosing a better tool can be found here.