As an example - Angular2 provides an excellent user experience, especially when tied to Bootstrap (v3 or v4) suitable for creating immersive experiences where the whole of the SharePoint chrome is replaced (other than the top bar).

However integrating it with the SharePoint REST API framework is cumbersome and unless extensive work is undertaken to mock SharePoint the unit testing and rapid deployment is very unpleasant if possible at all.

The SharePoint framework provides a rapid deployment and unit testable mechanism for developing SharePoint functionality but is limited to single web parts and extensions.

Please extend the model so that it can be used to create single page applications in the framework of the developers choice.

Besides the lack of detailed documentation around the REST API, given its importance as a major entry point for apps, it needs to be treated with more respect as a service offering. It's currently being updated in CUs and on SharePoint online without any notification of the changes being made (such as the $metadata endpoint appearing - was it in the August 2013 CU for on-prem?). We either need it to be released in a non-breaking change model (URL versioning such as the Twilio model), or messaging needs to be improved so we know when things are going to be changed. Having a log of changes that have been made to date would be helpful, along with understanding what changes are currently in the queue. For example, I would LOVE to know if the end game for the _api endpoint is to finally be a first-class OData v3 endpoint sometime soon.