Ruby on Rails is the most featurful and most popular MVC framework in Ruby for web application development. Merb is new, but attracts attention in Ruby community because of its focus on performance ahead of clever code, that Rails prefers in cost of performance. The basic Merb is just a small MVC core; modularity enables it to add features (Merb community chants an interesting slogan - "No code is faster than no code").
So, what will be next realease? A Rails one or Merb one? It will have both options.
1. For existing Rails developer, you can still get the all-in-one package
2. For existing Merb developers, you can download only the core. Later you can add feature as module
3. Improved architecture, and thus performance, for both framework.
4. Users of both camp will able to enjoy smooth migration to the next version