Improvements to the Plone REST API and integrating WebOb into the Zope stack are two different things with little overlap, as far as I can tell. Both are worth individual GSOC projects. REST API work could also include some work on the Zope Publisher and ZServer to support REST verbs, but the Request/Response object framework is not so important for REST.
WebOb integration for Zope was one of the topics we collected at: Summer of Code 2016
Thinking about the purpose of the WebOb proposal, I think it could help with a future transition to Pyramid and interoperability with WSGI. Anything else?