Here my comments:
1) Gulp and Grunt work
2) [Bug 443097] Ignore undefined HTML attributes, elements validation with extension point
I must find time to try to implement custom validator for AngularJS Eclipse. I think it's a good use case because angular has some complex cases.
3) Bug 482841 - JSDT to utilize org.eclipse.wst.validation.validatorV2 extension point
which is my hack to plug JSDT JavaEditor with WTP validator because JSDT is not a IStrcuturedModel.
IMHO, I think it should be very good to benefit with SSE validator for any editor even if the editor doesn't maintain a IStructuredModel.
4) JSON Editor
At first do you like the IJSONModel API with IJSONPair etc?
I think the main bugs you will find it's because sync of IJSONModel and editor is not robust. IMHO I think you should improve that (but it's a very hard task).
In other words I think you should add new tests and fix problem with sync.