The latter is already possible, and lots of plugins make use of it in one way or another. Basically, your helper app needs to start up a local websocket server, and UXP plugins can then connect to that. Once connected, you can send information back and forth quite rapidly.
We are looking at bringing some initial webview support to UXP, but it will first be limited to modal dialogs, and we still have a lot of security issues to iron out. (Webviews in our host applications are non-trivial.) There are a whole host of issues for supporting webviews in panels (keyboard focus is a big one!), and we’re also working through that. Furthermore, webviews do incur a fairly heavy memory and performance hit, especially if used in multiple panels.
It’s also worth nothing that a lot of plugins that used webviews in CEP also mixed that with Node.js – the latter is something UXP has no plans to support. If you need Node.js in your plugin, a helper app is the way to go.
Also coming this year is also a launchApplication API so that you can launch your helper app should it not be running. This should improve the user experience of helper apps greatly.