As for in-editor 3D modeling, I think artists will already have a specific program they have a preference for and use consistently like Blender or Maya which has lots of complex features they're already comfortable with and/or educated to use, and for people that don't need anything that complex, free in-editor tools already exist. That would definitely reduce the need for external tools and streamline basic asset creation/editing, plus reduce compatibility issues.
Support for vector graphics would be a nice touch (not for me personally at the moment. Image Editor, basic features, ability to write/import/export plugins/filters. Audio Editor, with a universal codec, basic features including recording, ability to extend. A patch-update from WebStorm 2016.1.1. Ive just installed JetBrains WebStorm 2016.1.1 Build WS-145.597 for evaluation on OS X latest. When I run JS files under the configuration, nothing is shown to the out put. There is already a predefined Node.js configuration which seems to work but in fact it doesnt. Code Editor, with at least code completion/suggestions and debugging/breakpoints/etc. You can find the full list of addressed issues in the release notes for the builds 145.1616 and 145.1503. I want to config WebStorm idle such that it can execute JS files just like Python idle which runs codes. What I would like to see as part of Unity Editor long term: any at all that's functional would be an improvement. As to which particular in-editor package that should be. It would definitely be great to code inside Unity and drop VS - as mentioned, it's a flow thing.