In a nutshell: the 1.6 release will be an API breaking release. All minor future releases will be similar; they mark the dawn of breaking API changes and new features will be developed on top of those. Critical- and security issues will be backported to the previous release (i.e. we will continue to support the last two versions).
- Whisper v5 should make its first alpha appearance in Geth v1.6.0. This may require some code modifications from people already using v2 of the protocol, so ideally we'd need either a documentation for v5, a transition document from v2->v5, or preferrably both.
- New flag --mode={full, light, fast}. Deprecate --fast, --light. Default --mode=fast.
- Optionally switch to a new logging system, retain the vmodule and have structured logging
- 1.6 no changes to the geth subcommand system. @basvk will write up a proposal for 1.7