One major change under the hood is that it will use the Mono project’s C# compiler-as-a-service to enable much better code completion and formatting. This allows the project to easily track the latest C# release, as well as improve syntax error checking and formatting in the environment. There is also a lot of Git version control support coming in, along with better views of version control. This means Git-specific commands such as push changes and switch/merge branches will work better, and log and blame output will be easier to work with. There will also be an inline disassembly viewer, which will automatically be displayed if you try and go to the source code of a method for which there is no source available.
Other less major (but still not insignificant) changes include .NET 4 project support, the ability for users to set custom policies across solutions and projects, catchpoint, tracepoint and conditional breakpoint support in the Windows debugger, and many more improvements.
It’s unclear as to when version 2.6 is going to be available as their roadmap has not been publically updated since May last year for version 2.4. You can check out the version’s plan on the Mono site.