1) Get rid of completions not available on Rhino
A lot of default completions presented are not available in Rhino, eg
things like AbortController, ActiveXObject, Animation, Audio, ...
How could I suppress such completions?
This is delegated to the language server (
https://github.com/theia-ide/typescript-language-server ), I don't know whether the language server can be configured to filter out some proposals or more simply to use a subset of the typical JS libraries. You'll need to check that.
If the language server doesn't allow it, then there is no solution; and Wild Web Developer doesn't plan to become a framework for custom JS editors, so I don't think you should expect such filtering capability to be provided by Wild Web Developer.
2) Add custom completions
Rhino allows to interact with Java classes, so I would like to add
completions to available packages and classes. This would require a
dynamic lookup done in Java. JSDT had extension points for code
completions. What would be the recommended way to achieve the same goal
in wild web developer?
Adding completion proposals can be achieved via extensions to the Generic Editor, which is used by Wild Web Developer.
3) EASE modules
EASE provides a custom way of loading script libraries. Is there a way
to add elements to the outline view?
The outline is built using language server response to workspace/documentSymbols request. So it seems more like a question about whether you can extend the LS directly.
Or, LSP4E uses the Common Navigator Framework to build outline. The language server is an extension to CNF; you can consider creating another CNF extension to add your specific elements to this tree.
