However I find the embedded scripting language idea interesting. It is possible to restrict the available classes in Nashorn with ClassFilter, but it is not available for Java 8 (so a Sponge client would have to bundle Java 8). I would argue that Groovy is nicer because it is already used for gradle and is similar enough to Java. It is possible to use a sandbox with it as well.
However, most likely if we do get to making a Sponge client API before custom GUIs and custom blocks/item IDs (which are pretty much the only things remaining, everything else like custom models for items and blocks are there) it will look like the current resource packs, with JSON and png files and everything else handled by the server. I don’t see much point for an embeddable scripting language for 95% of use cases.