Scala 3 will be a bigger undertaking, likely to have a major amount of breaking of core language contructs and other fundamental changes. Experimentation is already underway: it's called the Dotty compiler [1].
The changes as seen from the outside will be much more conservative than that. We're not ready to comment on specifics yet, but the main focus is to remove warts (deprecating them in 2.12/2.13 first where possible), make the type system and (thus) type inference more regular, and make safari^W scalac snappier.
The compiler internals and language concepts are being simplified substantially (we've learned a lot of interesting lessons in compiler engineering in the last decade, some of which require bigger changes).
The Scala 2.x series is for source compatible releases. The goal is that you can just recompile your Scala 2.11 code on Scala 2.12. (As was the case for the upgrade from 2.10 to 2.11 -- modulo some changes necessary for important bug fixes.)
We're reserving the bigger version bump for when source compatibility is not fully maintained. We take backwards compatibility very seriously, so the jump will be as small as we can make it (balanced with the need to keep evolving and improving the language). We'll share our thoughts on Scala's longer-term roadmap once they've crystallized (soon!).
[1]: https://github.com/lampepfl/dotty