Yes, we will have very little to do with requiring Spring data REST, mainly because at the moment the Jackson bindings are synchronous or blocking IO.
Getting a quick-win with the HAPI-JPA style app, that uses their existing libraries, with Spring data JPA should be our first pass with Cassandra as the database. This could possibly be done even before we begin the coding phase. Then we can try to make it reactive. @yashdsaraf , does that sound like a good plan to you for your GSoC project?