Зашел на github страницу RxJava и обнаружил, что вот уже месяц как существует третья версия! Пока что в статусе Release Candidate.

What's new:
- single dependency: Reactive-Streams
- continued support for Java 6+ & Android 2.3+
- fixed API mistakes and many limits of RxJava 2
- intended to be a replacement for RxJava 2 with relatively few binary incompatible changes
- Java 8 lambda-friendly API
- non-opinionated about source of concurrency (threads, pools, event loops, fibers, actors, etc.)
- async or synchronous execution
- virtual time and schedulers for parameterized concurrency
- test and diagnostic support via test schedulers, test consumers and plugin hooks

#rxjava

https://github.com/ReactiveX/RxJava/wiki/What's-different-in-3.0